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Transcript

VMware vSphere®
Storage APIs –
Array Integration (VAAI)
T E C H N I C A L M A R K E T I N G D O C U M E N TAT I O N
V 1 . 1 / D E C E M B E R 2 0 1 2
VMware vSphere Storage APIs –
Array Integration (VAAI)
T E C H N I C A L W H I T E P A P E R / 2
Table of Contents
Introduction to VAAI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
VAAI Block Primitives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Atomic Test & Set (ATS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
ATS Only Flag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
XCOPY (Extended Copy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Write Same (Zero) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
VAAI NAS Primitives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Full File Clone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Fast File Clone/Native Snapshot Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Extended Statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Reserve Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
VAAI Thin Provisioning Primitives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Thin Provisioning Stun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Thin Provisioning Space Threshold Warning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Dead Space Reclamation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
VAAI Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Pluggable Storage Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
VAAI Tuning and Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Checking VAAI Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Enabling and Disabling VAAI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
ATS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
XCOPY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
WRITE_SAME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
UNMAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Reverting to the Software Data Mover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Differences in VAAI Between vSphere 4.x and 5.x . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
VAAI Caveats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
VMFS Block Sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Raw Device Mappings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Sparse/Hosted Format VMDK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Misalignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
VMFS Extents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Different Destination Storage Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
About the Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
T E C H N I C A L W H I T E P A P E R / 3
VMware vSphere Storage APIs –
Array Integration (VAAI)
Introduction to VAAI
In a virtualized environment, storage operations traditionally have been expensive from a resource perspective.
Functions such as cloning and snapshots can be performed more efficiently by the storage device than by
the host.
VMware vSphere® Storage APIs – Array Integration (VAAI), also referred to as hardware acceleration or
hardware offload APIs, are a set of APIs to enable communication between VMware vSphere ESXi™ hosts and
storage devices. The APIs define a set of “storage primitives” that enable the ESXi host to offload certain storage
operations to the array, which reduces resource overhead on the ESXi hosts and can significantly improve
performance for storage-intensive operations such as storage cloning, zeroing, and so on. The goal of VAAI is to
help storage vendors provide hardware assistance to speed up VMware® I/O operations that are more efficiently
accomplished in the storage hardware.
Without the use of VAAI, cloning or migration of virtual machines by the vSphere VMkernel Data Mover involves
software data movement. The Data Mover issues I/O to read and write blocks to and from the source and
destination datastores. With VAAI, the Data Mover can use the API primitives to offload operations to the array if
possible. For example, if the desired operation were to copy a virtual machine disk (VMDK) file from one
datastore to another inside the same array, the array would be directed to make the copy completely inside the
array. Whenever a data movement operation is invoked and the corresponding hardware offload operation is
enabled, the Data Mover will first attempt to use the hardware offload. If the hardware offload operation fails,
the Data Mover reverts to the traditional software method of data movement.
In nearly all cases, hardware data movement will perform significantly better than software data movement. It
will consume fewer CPU cycles and less bandwidth on the storage fabric. Improvements in performance can be
observed by timing operations that use the VAAI primitives and using esxtop to track values such as CMDS/s,
READS/s, WRITES/s, MBREAD/s, and MBWRTN/s of storage adapters during the operation.
In the initial VMware vSphere 4.1 implementation, three VAAI primitives were released. These primitives applied
only to block (Fibre Channel, iSCSI, FCoE) storage. There were no VAAI primitives for NAS storage in this
initial release.
In vSphere 5.0, VAAI primitives for NAS storage and VMware vSphere Thin Provisioning were introduced.
VAAI Block Primitives
Atomic Test & Set (ATS)
In VMware vSphere VMFS, many operations must establish a lock on the volume when updating a resource.
Because VMFS is a clustered file system, many ESXi hosts can share the volume. When one host must make an
update to the VMFS metadata, a locking mechanism is required to maintain file system integrity and prevent
another host from coming in and updating the same metadata. The following operations require this lock:
1. Acquire on-disk locks
2. Upgrade an optimistic lock to an exclusive/physical lock
3. Unlock a read-only/multiwriter lock
4. Acquire a heartbeat
5. Clear a heartbeat
6. Replay a heartbeat
7. Reclaim a heartbeat
8. Acquire on-disk lock with dead owner
It is not essential to understand all of these operations in the context of this whitepaper. It is sufficient to
understand that various VMFS metadata operations require a lock.
T E C H N I C A L W H I T E P A P E R / 4
VMware vSphere Storage APIs –
Array Integration (VAAI)
ATS is an enhanced locking mechanism designed to replace the use of SCSI reservations on VMFS volumes
when doing metadata updates. A SCSI reservation locks a whole LUN and prevents other hosts from doing
metadata updates of a VMFS volume when one host sharing the volume has a lock. This can lead to various
contention issues when many virtual machines are using the same datastore. It is a limiting factor for scaling to
very large VMFS volumes. ATS is a lock mechanism that must modify only a disk sector on the VMFS volume.
When successful, it enables an ESXi host to perform a metadata update on the volume. This includes allocating
space to a VMDK during provisioning, because certain characteristics must be updated in the metadata to
reflect the new size of the file. The introduction of ATS addresses the contention issues with SCSI reservations
and enables VMFS volumes to scale to much larger sizes.
In vSphere 4.0, VMFS3 used SCSI reservations for establishing the lock, because there was no VAAI support in
that release. In vSphere 4.1, on a VAAI-enabled array, VMFS3 used ATS for only operations 1 and 2 listed
previously, and only when there was no contention for disk lock acquisitions. VMFS3 reverted to using SCSI
reservations if there was a multihost collision when acquiring an on-disk lock using ATS.
In the initial VAAI release, the ATS primitives had to be implemented differently on each storage array, requiring
a different ATS opcode depending on the vendor. ATS is now a standard T10 SCSI command and uses opcode
0x89 (COMPARE AND WRITE).
For VMFS5 datastores formatted on a VAAI-enabled array, all the critical section functionality from operations 1
to 8 is done using ATS. There no longer should be any SCSI reservations on VAAI-enabled VMFS5. ATS continues
to be used even if there is contention. On non-VAAI arrays, SCSI reservations continue to be used for
establishing critical sections in VMFS5.
ATS Only Flag
An ATS only flag appears on newly created VMFS5 datastores. When the host detects that the array supports
ATS on the device, the ATS only flag is written to the datastore. From that point on, ATS always will be used on
that datastore, so it can be used for all metadata operations.
The flag is not enabled, however, on VMFS5 volumes that were upgraded from VMFS3, facilitating seamless
upgrades from VMFS3 to VMFS5. This is because VMFS3 was not ATS complete, and during the upgrade
process, some hosts might continue to use SCSI reservations for certain metadata operations.
You cannot turn on the ATS only flag on VMFS5 volumes upgraded from VMFS3. The ATS only flag can be
manually enabled or disabled on a newly created VMFS5. To manually enable it, you can use the hidden option:
# vmkfstools –configATSOnly 1 [device path]
XCOPY (Extended Copy)
Without VAAI, a clone or migrate operation must use the VMkernel software Data Mover driver. If the files being
cloned are multiple GB in size, the operation might last for many minutes to many hours. This full copy consumes
host resources such as CPU cycles, DMA buffers and SCSI commands in the HBA queue.
This primitive requests that the array perform a full copy of blocks on behalf of the Data Mover. It primarily is
used in clone and migration operations (such as a VMware vSphere Storage vMotion® instance). ESXi hosts can
be configured to enable the EXTENDED COPY SCSI command. The SCSI opcode for XCOPY is 0x83.
T E C H N I C A L W H I T E P A P E R / 5
VMware vSphere Storage APIs –
Array Integration (VAAI)
Write Same (Zero)
When provisioning an eagerzeroedthick VMDK, a SCSI command is issued to write zeroes to disk blocks. Again,
this initialization request consumes host resources such as CPU cycles, DMA buffers and SCSI commands in the
HBA queue.
One of the most common operations on virtual disks is initializing large extents of the disk with zeroes to isolate
virtual machines and promote security. ESXi hosts can be configured to enable the WRITE_SAME SCSI
command to zero out large portions of a disk. This offload task will zero large numbers of disk blocks without
transferring the data over the transport link. The WRITE_SAME SCSI opcode is 0x93.
The following provisioning tasks are accelerated by the use of the WRITE_SAME command:
• Cloning operations for eagerzeroedthick target disks
• Allocating new file blocks for thin-provisioned virtual disks
• Initializing previous unwritten file blocks for zeroedthick virtual disks
NOTE: Some storage arrays, on receipt of the WRITE_SAME SCSI command, will write zeroes directly to disk.
Other arrays do not need to write zeroes to every location; they simply do a metadata update to write a page of
all zeroes. Therefore, it is possible that you will observe significant differences in the performance of this
primitive, depending on the storage array.
VAAI NAS Primitives
Prior to vSphere 5.0, VMware supported hardware acceleration on block storage devices. As of the vSphere 5.0
release, VMware has also included a number of new NAS hardware acceleration primitives. Unlike block
primitives, VAAI NAS primitives will be available only through the use of a NAS vendor plug-in, which is provided
by your storage array vendor.
Full File Clone
Full File Clone, or Full Copy, is similar to the Extended Copy (XCOPY) hardware acceleration primitive provided
for block arrays. This primitive enables virtual disks to be cloned by the NAS device rather than by using the
Data Mover, which consumes ESXi host CPU and memory resources as well as network bandwidth. There is one
noticeable difference between this primitive and the XCOPY primitive on block storage: This primitive cannot be
called for Storage vMotion operations, which continue to use the Data Mover. Only virtual machines that are
powered off and then migrated can offload to the storage array by using this primitive. The Storage vMotion
restriction aside, the benefit is that cold clones or “deploy from template” operations can be offloaded to the
storage hardware, and this reduces the resource consumption of the ESXi host.
Fast File Clone/Native Snapshot Support
Fast File Clone enables the creation of virtual machine snapshots to be offloaded to an NAS storage array.
This is a primitive that forms the basis of VMware View Composer™ Array Integration (VCAI), which was released
as a technical preview in VMware View™ 5.1. VMware View Composer can elect to have desktops based on linked
clones created directly by the storage array rather than by using ESXi host CPU and memory resources
for the same task. In vSphere 5.1, this primitive has been extended to VMware vCloud® vApps™, whereby
VMware vCloud Director™ can elect to have vCloud vApps based on linked clones instantiated by the storage
array rather than the ESXi host.
T E C H N I C A L W H I T E P A P E R / 6
VMware vSphere Storage APIs –
Array Integration (VAAI)
Extended Statistics
This primitive enables visibility into space usage on NAS datastores. This is especially useful for thin-provisioned
datastores because it enables vSphere to display actual space usage statistics in situations where
oversubscription was used. Previously, vSphere administrators needed to use array-based tools to manage and
monitor how much space a thinly provisioned VMDK was consuming on a back-end datastore. With this new
primitive, this information can be surfaced up in the VMware vSphere Client™. This enables vSphere
administrators to have a much better insight into storage resource consumption, without the need to rely on
third-party tools or engage with their storage array administrators.
Reserve Space
In previous versions of vSphere, only a thin VMDK could be created on NAS datastores. This new reserve space
primitive enables the creation of thick VMDK files on NAS datastores, so administrators can reserve all of the
space required by a VMDK, even when the datastore is NAS. As shown in Figure 1, users now have the ability to
select lazy-zero or eager-zero disks on NAS datastore.
Figure 1. Thick-Disk Provisioning on NFS Datastores
VAAI Thin Provisioning Primitives
In environments where thin provisioning is used in the management and monitoring of datastores, there are
frequent complaints that the array is not informed that blocks have become free when virtual machines have
been deleted or migrated from a datastore. This leads to array management tools’ reporting much higher space
consumption than is actually the case. Out-of-space (OOS) concerns are also common. In the past, OOS
conditions could impact all virtual machines on the OOS thin-provisioned datastore.
To address these concerns, multiple enhancements were made to VAAI in vSphere 5.0. In particular, several new
primitives have been added to better support Thin Provisioning.
T E C H N I C A L W H I T E P A P E R / 7
VMware vSphere Storage APIs –
Array Integration (VAAI)
Thin Provisioning Stun
The first enhancement was introduced to address concern regarding the impact on virtual machines when
thin-provisioned datastore usage reaches 100 percent of capacity. Historically, this affected all virtual machines
running on the datastore. With the release of the vSphere 5.0 VAAI Thin Provisioning primitives, if a thin-provisioned
datastore reaches 100 percent usage, only those virtual machines requiring extra blocks of storage space are
paused; those not needing additional space continue to run. After additional space is allocated to the
thin-provisioned datastore, the paused virtual machines can be resumed.
Thin Provisioning Space Threshold Warning
vSphere cannot detect how much space a thin-provisioned datastore is consuming at the back end. The first
indication a vSphere administrator has that an overcommitted datastore has no available space is that no more
virtual machines can be provisioned and other virtual machines stop running. With this new primitive, a warning
is raised and surfaced in VMware vCenter™ via VAAI if a thin-provisioned datastore reaches a specific threshold,
as is shown in Figure 2. The threshold values are set on the array, if there is a means to do so, and are defined by
the storage administrator; vSphere does not set this threshold. This enables an administrator to proactively add
more storage, extend the datastore, or run a Storage vMotion operation to migrate some virtual machines off of
the datastore to avoid OOS conditions.
Figure 2. Thin Provisioning Out-of-Space Warning
Dead Space Reclamation
A new VAAI primitive, using the SCSI UNMAP command, enables an ESXi host to inform the storage array that
space can be reclaimed that previously had been occupied by a virtual machine that has been migrated to
another datastore or deleted. This enables an array to accurately report space consumption of a thin-provisioned
datastore and enables users to monitor and correctly forecast new storage requirements.
The mechanism for conducting a space reclamation operation has changed somewhat since the primitive was
introduced in vSphere 5.0. Then, the operation was automatic: When a virtual machine was migrated from a
datastore or deleted, the UNMAP was called immediately and space was reclaimed on the array. There were
some issues with this approach, however, primarily regarding performance and an array’s ability to reclaim the
space in an optimal time frame. For this reason, the UNMAP operation is now a manual process. Because it is the
only manually initiated primitive, the process to reclaim dead space will be discussed in detail.
To reclaim dead space from a thin-provisioned datastore in vSphere 5.1, first verify that the storage array is
capable of processing the SCSI UNMAP command. With esxcli, users can display device-specific details
regarding Thin Provisioning and VAAI.
T E C H N I C A L W H I T E P A P E R / 8
VMware vSphere Storage APIs –
Array Integration (VAAI)
# esxcli storage core device list –d naa.xxx
naa.xxx
Display Name: NETAPP Fibre Channel Disk (naa.xxx)
Has Settable Display Name: true
Size: 51200
Device Type: Direct-Access
Multipath Plugin: NMP
Devfs Path: /vmfs/devices/disks/naa.xxx
Vendor: NETAPP
Model: LUN
Revision: 8020
SCSI Level: 4
Is Pseudo: false
Status: on
Is RDM Capable: true
Is Local: false
Is Removable: false
Is SSD: false
Is Offline: false
Is Perennially Reserved: false
Thin Provisioning Status: yes
Attached Filters: VAAI_FILTER
VAAI Status: supported
Other UIDs: vml.xxx
The device is indeed thin provisioned and supports VAAI. Next, users can display the VAAI primitives supported
by the array for that device, as well as whether the array supports the UNMAP primitive for dead space
reclamation (referred to as the Delete Status). Another esxcli command is used for this step, as is shown in
the following:
# esxcli storage core device vaai status get -d naa.xxx
naa.xxx
VAAI Plugin Name: VMW_VAAIP_NETAPP
ATS Status: supported
Clone Status: supported
Zero Status: supported
Delete Status: supported
The device displays Delete Status: supported to indicate that it is capable of sending SCSI UNMAP commands
to the array when a space reclamation operation is requested.
If a Storage vMotion operation is initiated and a virtual machine is moved from a source datastore to a
destination datastore, the following actions will reclaim that space: When the Storage vMotion operation has
completed, the vSphere Client will report that the VMFS5 volume now has much more free space. The issue is
that when the amount of free space is checked on the thin-provisioned LUN backing this VMFS5 volume on the
storage array, there is unused and stranded space. In this example, a NetApp FAS array is providing the
back-end storage. Using a lun show CLI command on this NetApp array, we see in the following that 8.8GB of
space is still being consumed, even though there are now no virtual machines or any other files on this datastore.
T E C H N I C A L W H I T E P A P E R / 9
VMware vSphere Storage APIs –
Array Integration (VAAI)
lun show -v /vol/vol2/thin-lun
/vol/vol2/thin-lun 50g (53687091200) (r/w, online, mapped)
Serial#: xxx
Share: none
Space Reservation: disabled
Multiprotocol Type: vmware
Maps: unmap=51 issi=51
Occupied Size: 8.8g (9473908736)
Creation Time: Tue Apr 24 15:16:52 BST 2012
Cluster Shared Volume Information: 0x0
This is the crux of the issue that we are trying to solve with the VAAI UNMAP primitive. To run the UNMAP
command, change the directory to the root of the VMFS volume that you want to reclaim space from. The
following command is run:
vmkfstools –y <% of free space to unmap>
In this example, we attempted a reclamation of 60 percent of free space. The vmkfstools -y command displays
the following:
Attempting to reclaim 60% of free capacity 48.8 GB (29.3 GB) on VMFS-5 file system
‘source-datastore’ with max file size 64 TB.
Create file .vmfsBalloontsWt8w of size 29.3 GB to reclaim free blocks.
Done.
vmkfstools -y created a balloon file of 29.3GB, which is 60 percent of the free capacity (48.8GB). This
temporary balloon file is equal to the size of the space to be unmapped/reclaimed.
NOTE: Caution – If a percentage value in the high 90s or 100 is specified, the temporary balloon file that is
created during the reclamation operation might fill up the VMFS volume. Any growth of current VMDK files due
to running virtual machines writing to their disks or the creation of new files, such as snapshots, might fail due
to unavailable space. Care should be taken when calculating the amount of free space to reclaim.
Finally, query the status of the thin-provisioned LUN on the storage array. It should show a difference in
Occupied Size, as shown in the following:
lun show -v /vol/vol2/thin-lun
/vol/vol2/thin-lun 50g (53687091200) (r/w, online, mapped)
Serial#: xxx
Share: none
Space Reservation: disabled
Multiprotocol Type: vmware
Maps: unmap=51 issi=51
Occupied Size: 76.3m (79966208)
Creation Time: Tue Apr 24 15:16:52 BST 2012
Cluster Shared Volume Information: 0x0
The dead space successfully has been reclaimed for the thin-provisioned datastore; the array now can correctly
report the space usage.
Disabling the UNMAP primitive does not affect any of the other Thin Provisioning primitives such as
Thin Provisioning Stun and the space threshold warning. All primitives are orthogonal.
VMware vSphere Storage APIs –
Array Integration (VAAI)
VAAI Implementation
Pluggable Storage Architecture
There are several components required in the VMkernel to make VAAI work correctly. The first of these is the
Pluggable Storage Architecture (PSA) device filter framework. This is a feature that is intended to facilitate
device-level value-add into the PSA I/O stack. It enables VMware to introduce new value-add into the PSA stack
asynchronously, decoupling it from the vSphere release cycle. The primary motivation is to support VAAI.
Without the PSA device filter framework, VAAI would have to be implemented in the storage array type policy
(SATP) on a per-array basis. Implementation in the PSA device filter framework reduces duplicate code and
saves memory space in the VMkernel. Also, if VAAI were implemented in the SATP, it would prevent other
third-party plug-ins, such as EMC PowerPath, from using the offload primitives.
The term “filter” is somewhat misleading because the PSA device filter framework enables specific value-added
software to be inserted between VMFS and the PSA device layer.
The second required component can be referred to as a VAAI plug-in specific to the VAAI filter. It implements
vendor-specific VAAI functions such as ATS, XCOPY and WRITE_SAME. There were different implementations
of the VAAI block primitives in vSphere 4.1, but all of the primitives in vSphere 5.0 have been ratified by T10, so
any array that is T10 compliant should be able to use VAAI.
VMFS Layer
PSA Layer
NMP PowerPath
PSA Path
to Device
Veritas DMP etc.
SATP PSP
PSA Device
Filter
VAAI
Filter
Figure 3. Pluggable Storage Architecture
To leverage VAAI functionality on a LUN, you must have both a PSA device filter and a vendor-specific VAAI
plug-in for each device. VMware has a single PSA device filter plug-in called VAAI_FILTER. If a device supports
VAAI offloads, it first will be claimed by the VAAI filter, VAAI_FILTER. After it has been claimed by the filter, a
vendor-specific VAAI plug-in will claim and then manage the device.
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The following commands display the claim rules that are associated with device filters in the PSA.
By using the following command, you can check whether an individual device has been claimed by the VAAI
filter. As is shown in the following, the Attached Filters and VAAI Status fields are located toward the end of
the output:
~ # esxcli storage core device list -d naa.xxx
naa.xxx
Display Name: NETAPP iSCSI Disk (naa.xxx)
Has Settable Display Name: true
Size: 20480
Device Type: Direct-Access
Multipath Plugin: NMP
Devfs Path: /vmfs/devices/disks/naa.xxx
Vendor: NETAPP
Model: LUN
Revision: 8020
SCSI Level: 4
Is Pseudo: false
Status: degraded
Is RDM Capable: true
Is Local: false
Is Removable: false
Is SSD: false
Is Offline: false
Is Perennially Reserved: false
Thin Provisioning Status: yes
Attached Filters: VAAI_FILTER
VAAI Status: supported
Other UIDs: vml.xxx
If a device supports VAAI and has been claimed successfully by the VAAI filter, the filter will appear in the
plug-in list command:
~ # esxcli storage core plugin list
Plugin name Plugin class
---------------- ------------
VMW_VAAIP_NETAPP VAAI
VAAI_FILTER Filter
NMP MP
In this example, the VAAI filter and the vendor-specific VAAI plug-in are displayed.
Finally, another CLI command can be used to verify which vendor-specific VAAI plug-in has claimed a device.
This command shows both the VAAI plug-in name and the primitives that it supports.
~ # esxcli storage core device vaai status get -d naa.xxx
naa.xxx
VAAI Plugin Name: VMW_VAAIP_NETAPP
ATS Status: supported
Clone Status: supported
Zero Status: supported
Delete Status: supported
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VMware vSphere Storage APIs –
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VAAI Tuning and Monitoring
This section describes some of the options available for fine-tuning and monitoring the VAAI primitives. This will
help administrators determine whether VAAI is working correctly in their respective environments.
We first will discuss XCOPY. The default XCOPY size is 4MB. With a 32MB I/O, the expectation would be to see
this counter in esxtop incrementing in batches of eight, the number of work items that will be created to deliver a
32MB I/O. The default XCOPY size can be incremented to a maximum value of 16MB if required, but that should
be done only on the advice of your storage array vendor. The following command shows how to query the
current transfer size and how to change it:
# esxcfg-advcfg -g /DataMover/MaxHWTransferSize
Value of MaxHWTransferSize is 4096
# esxcfg-advcfg -s 16384 /DataMover/MaxHWTransferSize
Value of MaxHWTransferSize is 16384
NOTE: Please use extreme caution when changing this advanced parameter. This parameter is a global
parameter, so it will impact ALL storage arrays attached to the host. While a storage array vendor might
suggest making a change to this parameter for improved performance on their particular array, it may lead to
issues on other arrays which do not work well with the new setting, including degraded performance.
The data-out buffer of the WRITE_SAME command contains all zeroes. A single zero operation has a default
zeroing size of 1MB. The maximum number of outstanding WRITE_SAME commands is 32.We currently do not
support changing the WRITE_SAME size of 1MB.
One can observe the clone operations (XCOPY), ATS operations, and zero operations (WRITE_SAME) in an
esxtop output. The following is an esxtop output showing the various block primitives:
FIgure 4. esxtop Output – Block Primitives
To see this output in esxtop, first select u for device view, and then f to change which fields are displayed.
Options o and p display VAAI statistics. Each primitive also has a column for failure statistics, that is, CLONE_F,
ATSF and ZERO_F. These should be monitored for any failed operations.
One side effect is that VAAI offloads can lead to distorted kernel average latencies (KAVGs). When VAAI
commands are issued via the VAAI filter, there actually are two commands sent. These are top-layer commands
that are issued but never sent to the actual device (they stay within the VMkernel). These commands are
intercepted by the VAAI filter and the VAAI plug-in and are replaced by the vendor-specific commands, which
are issued to the device.
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For this reason, esxtop shows device statistics for only the top-level commands. As a result, the values for
latencies seem unusual when compared to results obtained when VAAI is not enabled.
FIgure 5. esxtop Output – Latency Values
In this—and only this—instance, the latencies observed in esxtop should not be interpreted as a performance
issue, unless there are other symptoms present.
Checking VAAI Support
To check VAAI status, a number of esxcli commands are available. For example, to see which primitives are
supported for a particular device, the following command can be used:
# esxcli storage core device vaai status get -d naa.xxx
naa.xxx
VAAI Plugin Name: VMW_VAAIP_NETAPP
ATS Status: supported
Clone Status: supported
Zero Status: supported
Delete Status: supported
In the preceding output, the ATS Status, Clone Status (XCOPY), Zero Status (WRITE_SAME) and Delete Status
(UNMAP) are supported. The vSphere UI also can be used to verify VAAI support. The overall VAAI status is
displayed as Supported, Not supported or Unknown under Hardware Acceleration in the UI, as is shown in
the following:
FIgure 6. VAAI Status
Hardware acceleration status is determined by the support state of various primitives. If ATS is supported, the
UI displays Hardware Acceleration as Supported. If, however, ATS, XCOPY and zero all are unsupported,
Hardware Acceleration displays as Unsupported. All other support states set the hardware acceleration status
to Unknown, which typically is a state that is displayed until the first clone and zero operation is initiated. Many
users initiate an eagerzeroedthick clone of a file to test the VAAI status.
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Enabling and Disabling VAAI
To disable some of the VAAI primitives—for troubleshooting purposes, for example—see the following CLI
commands, which detail how to enable and disable VAAI primitives.
ATS
To check the status of the ATS primitive and to turn it on and off at the command line, the following commands
can be used:
# esxcli system settings advanced list --option /VMFS3/HardwareAcceleratedLocking
Value of HardwareAcceleratedLocking is 1
# esxcli system settings advanced set --int-value 0 --option /VMFS3/
HardwareAcceleratedLocking
# esxcli system settings advanced list --option /VMFS3/HardwareAcceleratedLocking
Value of HardwareAcceleratedLocking is 0
XCOPY
To turn the Extended Copy primitive for cloning on and off, use the previous command with the following
advanced setting:
/DataMover/HardwareAcceleratedMove
WRITE_SAME
To turn the Write Same primitive for zeroing blocks on and off, use the following advanced setting:
/DataMover/HardwareAcceleratedInit
UNMAP
To turn the UNMAP primitive for space reclamation on and off, use the following advanced setting:
/VMFS3/EnableBlockDelete
In vSphere 4.1, it was possible to define how often an ESXi host verified whether hardware acceleration was
supported on the storage array.
# esxcfg-advcfg -g /DataMover/HardwareAcceleratedMoveFrequency
Value of HardwareAcceleratedMoveFrequency is 16384
This means that if at initial deployment, an array does not support the offload primitives, but at a later date the
firmware on the arrays is upgraded and the offload primitives become supported, nothing must be done
regarding ESXi—it automatically will start to use the offload primitives. The value relates to the number of I/O
requests that occur before another offload is attempted. With an I/O size of 32MB, 512GB of I/O must flow
before a VAAI primitive is retried.
The same is true for an offload failure. If the array returns a failure status for an offload operation, the software
Data Mover is used to continue the operation. After another 16,384 I/O requests using the software Data Mover,
the VAAI offload is once again attempted.
HardwareAcceleratedMoveFrequency exists only in vSphere 4.1. In vSphere 5.0 and later, the parameter was
replaced with a periodic VAAI state evaluation that runs every 5 minutes.
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VMware vSphere Storage APIs –
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Reverting to the Software Data Mover
Before VAAI, migrations of virtual machines, and their associated disks, between datastores was done by a
component called the software Data Mover, a loadable VMkernel module accessible via special library calls. A
data move request, in the form of a data structure, is submitted to the Data Mover for asynchronous completion.
The Data Mover takes this data structure and divides it into smaller I/O requests that it sends to the appropriate
back end.
The Data Mover relies on a constantly filled queue of outstanding I/O requests to achieve maximum throughput.
Incoming Data Mover requests are broken down into smaller chunks, and asynchronous I/Os are simultaneously
issued for each chunk until the configured Data Mover queue depth is filled. Then the asynchronous Data Mover
request returns to the caller who would normally proceed with what it was doing or wait for I/O completion.
Meanwhile, as and when an asynchronous request completes, the next logical request for the overall Data Mover
task is issued, whether it is for writing the data that was just read or to handle the next chunk.
In the following example, a clone of a 64GB VMDK file is initiated. The Data Mover is asked to move data in terms
of 32MB transfers. The 32MB is then transferred in “PARALLEL” as a single delivery but is divided up into a much
smaller I/O size of 64KB, using 32 work items that are all queued in parallel (because they execute
asynchronously). To transfer this 32MB, a total of 512 I/Os of size 64KB are issued by the Data Mover. By
comparison, a similar 32MB transfer via VAAI uses eight work items, because XCOPY uses 4MB transfer sizes.
VMkernel can choose from the following three Data Mover types:
• fsdm – This is the legacy Data Mover, the most basic version. It is the slowest because the data moves all the
way up the stack and then down again.
• fs3dm (software) – This is the software Data Mover, which was introduced with vSphere 4.0 and contained
some substantial optimizations whereby data does not travel through all stacks.
• fs3dm (hardware) – This is the hardware offload Data Mover, introduced with vSphere 4.1. It still is the fs3dm,
but VAAI hardware offload is leveraged with this version. fs3dm is used in software mode when hardware
offload (VAAI) capabilities are not available.
The following diagram shows the various paths taken by the various Data Movers:
hw ooad
fs3dm
fsdm
Application
Hypervisor
Fabric
ArrayLUN
01
LUN
02
FIgure 7. Data Mover Types
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The decision to transfer using the software Data Mover or using hardware offloading to the array with VAAI is
taken upfront by looking at storage array hardware acceleration state. If we decide to transfer using VAAI and
then FAIL—a scenario called degraded mode—the VMkernel will try to accomplish the transfer using the
software Data Mover. It should be noted that the operation is not restarted; rather it picks up from where the
previous transfer left off as we do not want to abandon what could possibly be very many gigabytes of copied
data because of a single transient transfer error.
When reverting to the software Data Mover, we cannot operate in terms of the XCOPY transfer size of 4MB, due
to memory buffer requirement, so the transfer size is reduced to 64KB.
Differences in VAAI Between vSphere 4.x
and 5.x
There are a number of differences between the first phase/release of VAAI in vSphere 4.1 and the second phase
of VAAI, which was released with vSphere 5.0. The following is a list of those differences:
• VAAI in 5.0 now uses standard T10 primitives rather than per-array manufacturer commands.
• There is now full ATS support for all metadata operations with VMFS5.
• vSphere 5.0 introduced support for NAS primitives, including VCAI.
• vSphere 5.0 introduced support for Thin Provisioning primitives, including UNMAP.
• VMware HCL now requires performance of primitives before an array receives VAAI certification. This is
important because certain storage arrays that appear in the vSphere 4.1 HCL might not appear in the
vSphere 5.0 HCL if the performance of the offload primitives does not meet requirements.
VAAI Caveats
In cases involving the following known caveats related to VAAI, hardware offloads to the array will not be
leveraged; the Data Mover will be used instead:
VMFS Block Sizes
If the source and destination VMFS volumes have different block sizes, data movement will revert to the generic
FSDM layer, which will move onlysoftware data.
Raw Device Mappings
If the source file type is raw device mapping (RDM) and the destination file type is non-RDM (regular file),
migration occurs via the Data Mover.
Sparse/Hosted Format VMDK
If either the source or destination VMDK involved in the operation is of sparse or hosted format, the Data Mover
is used.
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VMware vSphere Storage APIs –
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Misalignment
The software Data Mover is used if the logical address and/or transfer length in the requested operation is not
aligned to the minimum alignment required by the storage device.
If the VMFS datastores are created using vSphere Client or vCenter, it is likely that all requests will be aligned. If a
LUN is incorrectly partitioned using low-level tools such as fdisk or partedUtil, the operation might revert to
using the software Data Mover.
Because customers typically use vSphere Client and vCenter exclusively for a created datastore, it is unlikely that
this issue would be encountered. If a customer’s environment was migrated from ESX 2.x/VMFS2 in the distant
past, however, the volumes might be misaligned.
This can be an issue on some VAAI arrays that have a high alignment requirement.
VMFS Extents
VMware supports VAAI primitives on VMFS with multiple LUNs/extents, if they all are on the same array and the
array supports offloading. There are some limitations on how ATS can be used on multiextent VMFS volumes,
but that impact is relatively low.
On VAAI Hardware New VMFS5 Upgraded VMFS5 VMFS3
Single-extent
datastore
ATS only 1 ATS, but reverts to
SCSI-2 reservations
ATS, but reverts to
SCSI-2 reservations
Multiextent
datastore
Allows spanning
only on ATS
hardware 2
ATS, except when
locks on non-head
ATS, except when
locks on non-head
Table 1. ATS Limitations on Multiextent VMFS Volumes
Different Destination Storage Array
VMware does not support VAAI primitives on VMFS with multiple LUNs/extents if they all are on different arrays,
even if all arrays support offloading. Hardware cloning between arrays—even if it is within the same VMFS
volume—will not work, so that would revert to software data movement.
1. If a new VMFS5 is created on a non-ATS storage device, SCSI-2 reservations will be used.
2. When creating a multiextent datastore in which ATS is used, the VMware vCenter Server™ will filter out non-ATS devices, so only devices that support
the ATS primitive can be used.
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VMware vSphere Storage APIs –
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Acknowledgments
Thank you to James Senicka, Paudie O’Riordan, Duncan Epping, Tejasvi Aswathanarayana and Ilia Sokolinski for
reviewing the contents of this paper.
About the Author
Cormac Hogan is a senior technical marketing architect with the Cloud Infrastructure Product Marketing group
at VMware. He is responsible for storage in general, with a focus on core VMware vSphere storage technologies
and virtual storage, including the VMware vSphere Storage Appliance. He has been in technical marketing
since 2011.
• Follow Cormac’s blogs at http://blogs.vmware.com/vsphere/storage and http://cormachogan.com.
• Follow Cormac on Twitter: @VMwareStorage.
VMware, Inc. 3401 Hillview Avenue Palo Alto CA 94304 USA Tel 877-486-9273 Fax 650-427-5001 www .vmware .com
Copyright © 2012 VMware, Inc . All rights reserved . This product is protected by U .S . and international copyright and intellectual property laws . VMware products are covered by one or more patents listed
at http://www .vmware .com/go/patents . VMware is a registered trademark or trademark of VMware, Inc . in the United States and/or other jurisdictions . All other marks and names mentioned herein may be
trademarks of their respective companies . Item No: VMW-TWP-vSPHR-STOR-VAAI-USLET-102 Docsouce: OIC-12VM008 .06

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