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Reference Architectures

• Pure Storage FlashStack Reference Architecture with Citrix XenDesktop 7.7 on //m20
• Pure Storage FlashStack Reference Architecture with XenDesktop 7.6.1 on vSphere 6.0
• Pure Storage Reference Architecture with Citrix XenDesktop 5.6

General VDI Sizing Guidelines

SCSI UNMAP

Deleting a file on a host file system does not automatically free up block space on storage systems. To do this, you need to trigger SCSI_UNMAP in order to notify the storage vendor that space should be freed up. This is required for many storage solutions, including Pure Storage FlashArrays. An UNMAP allows an operating system to inform an SSD which blocks of data are no longer considered in use and so can be wiped. This process informs the FlashArray what space can be reclaimed and ensures that you are not consuming unnecessary space.  For example, if a FlashArray consistently reports more data on a volume than host file-system utilities report for the file system, UNMAP is probably not being run regularly enough.

⁽¹⁾ The VAAI primitive UNMAP only works where the partition offset is a multiple of 1 MB. Operation will fail on misaligned VMFS-3 datastores or misaligned VMFS-3 datastores that have been converted to VMFS-5.

VMware Guest OS Space Reclamation

Many file systems cannot trigger the SCSI_UNMAP primitive like UFS, JFS, JFS2, NTFS-w2k8, etc. Additionally, some of the recent ones that can perform SCSI_UNAMP natively (Windows 2008 R2, Windows 2012, EXT4, etc) still have trouble doing so when operating within a guest VM, unless you are using Raw Device Mapping (RDM).

VMware VMFS - UNMAP Outside of the Guest

To reclaim unused storage blocks on a VMFS datastore, use the following VMware processes:

• Space Reclamation in VMWare ESXi 5.5
  NOTE : As noted in the VMware Best Practices page, make sure the UNMAP block count is set to 1% of the VMFS free space.

• Space Reclamation in VMWare ESXi 5.0/5.1
  NOTE : When running vmkfstools in 5.0/5.1 the process creates a balloon file that consumes space.  We recommend that you reclaim 20% at a time using this command and do not process more than 2TB of reclaimed space at a time.

Reclaiming Space within the Guest

vSphere 5.0, 5.1, and 5.5

In order to reclaim space inside of a virtual disk, we recommend that you use of a zeroing tool inside of the guest. See the following KB articles for reclaiming space:

• Windows: Use sdelete to reclaim space.
• Linux: Using shred to zero out the free space.

vSphere 6.0

With the introduction of vSphere 6.0, the EnableBlockDelete option was changed so that it enables VMFS block delete when UNMAP is issued from a guest operating system. That is, if you enable this option, ESXi will now permits guest operating systems to issue UNMAP commands to a virtual disk to be translated down to the FlashArray so that the space can be reclaimed. Beyond that, if the virtual disk is thin, it will be reduced in size by the amount of space that was reclaimed.

Note : Apparently only thin virtual disks support this guest-UNMAP functionality.

vSphere 6.5

The release of vSphere 6.5 supports Linux-based In-Guest UNMAP.

Executing In-Guest UNMAP/TRIM with Linux

There are a three options for reclaiming space as guest in vSphere 6.5 using Linux:

• Mount the file system with the discard option: mount -t ext4 -o discard /dev/sdc /mnt/UNMAP. This directs the system to automatically issue UNMAP when files are deleted from the file system.
• Run the command sg_unmap . It allows you to run UNMAP on specific Logical Block Addresses (LBAs).
• Run the command fstrim . It reclaims dead space across a directory or entire file system on demand. It does not require the discard option to be set, but is compatible with file systems that do have it enabled.

Trusted commentary on this subject recommends the discard option is the best option. The command sg_unmap requires extensive manual work and familiarity with logical block address placement and the command fstrim can run into alignment issues.

References

http://www.codyhosterman.com/2015/04...n-vsphere-6-0/

https://www.codyhosterman.com/2016/11/whats-new-in-esxi-6-5-storage-part-i-unmap/

Symptoms

While attempting to install the Pure Storage vSphere Plugin, specifically while using "vir1" as the primary interface, the following error message is received:

CLI error:

pureplugin: error: No virtual IP configured.

GUI error:

This issue is not seen when the 'vir0' interface is configured, enabled, and on the same network as the vCenter Server.

Diagnosis

There is currently a limitation with our vSphere Plugin that doesn't allow you to install / register it on the vCenter Server using the 'vir1' interface. You must have the 'vir0' interface enabled, and configured, on the vCenter Server network to get the registration process to complete successfully.

Solution

Enable & configure 'vir0' on the FlashArray with a valid IP, netmask, & gateway to ensure successful installation of the Pure Storage vSphere plugin. You can refer to TI-3866 for additional information on this specific issue.

This KB will be updated once we have a working vSphere Plugin that addresses this issue. Until then, please perform the steps above to work around this issue.

Symptoms

You present new LUN(s) to the ESXi Host / Cluster from the Pure Storage FlashArray. Upon initiating a rescan you verify that the expected LUN(s) are not seen on the ESXi Host as available devices under the "Storage Adapters" section. While investigating the issue you are able to confirm other LUNs are successfully mapped to the ESXi Host without issue.

Note: If checking from the command line of the ESXi Host the LUN(s) would be missing the following command:

'esxcli storage core device list'

Diagnosis

Upon reviewing the ESXi Host 'vmkernel' logs you are able to locate the following error message being reported at the time of rescan:

2016-04-23 22:21:43.483Z cpu13:34281087)WARNING: ScsiPath: 903: The number of paths allocated has reached the maximum: 1024. Path: vmhba2:C0:T14:L0 will not be allocated. This warning won't be repeated until some paths are removed.

Additionally, when running the following command from the ESXi Host command line interface it reports a value of '1024':

grep vmhba esxcfg-mpath -b |wc -l

Solution

The error message above indicates that the the maximum number of total paths allowed to an ESXi Host has been reached. You will not be allowed to add additional LUN(s) to the ESXi Host until this is addressed.

This can be resolved the following ways:

• Reduce the number of paths to each LUN so they can add the additional LUN(s) as needed.
• Reduce the number of total LUNs (if possible) to the ESXi Host so they can have desired number of paths.
• Map the LUNs to a new ESXi Host / Cluster.

This decision needs to be made by the customer to determine what the best scenario for their environment is. We do not have control over this variable within the ESXi Host. If they have additional directions kindly direct them to VMware Support as needed.

Review the following VMware links below for additional information:

• Maximum number of paths reached (1020654)
• VMware vSphere 5.1 Maximum Configurations - Page 3
• VMware vSphere 5.5 Maximum Configurations - Page 3
• VMware vSphere 6.0 Maximum Configurations - Page 12