Archive: Posts

VMware View Storage Accelerator (VSA), a.k.a. Content Based Read Cache (CBRC), versus Virtunet VirtuCache

VSA caches only those blocks that are shared by all VDI VMs, and it can only use a maximum of 2GB host RAM. Because of these two reasons, it ends up caching only a small subset of blocks from the master VM 1. In comparison, VirtuCache caches all storage IO, both reads and writes, whether it is from master VM, end user VDI VMs, server VMs, ESXi kernel, and it can cache to large amounts of in-host SSD and/or RAM. These two aspects of VirtuCache ensure that almost all storage IO is serviced from in-host cache media.

More details in the table below.

Feature

VirtuCache

View Storage Accelerator

Host side caching for VMware.

Yes.

Yes.

Works for server and VDI VMs.

Yes.

Works only with Horizon View VDI. Does not work with server VMs or non-VMware VDI like Xen Desktop.

Caches to RAM.

Yes. No restriction on amount of RAM that can be used.

Yes. Can cache to a maximum of 2GB RAM only.2

Caches to SSD.

Yes. No restriction on amount of SSD capacity that can be used. If both RAM and SSD are used, then the cache is tiered between RAM and SSD.

No.2

Caches Reads.

Yes. All reads from VMs (Server and VDI) and ESXi kernel are cached.

Only data that is shared across all VMs (common blocks) are cached, so only a subset of data (reads) from master VM is cached.3

Caches Writes.

Yes. All recent writes from VMs (Server and VDI) and ESXi kernel are cached.

No.2

Admin Overhead, etc.

Assign caching policy to VMs or Datastores. No ongoing administration required after.

Requires periodic ‘Regeneration’ of index, during which time VMs might be slow or inaccessible.4

VMs > 512GB are not supported.5

Turning on VSA requires recompose.

VMware authored cross-references.

1 – p39 of https://docs.vmware.com/en/VMware-Horizon-7/7.4/horizon-architecture-planning.pdf

2 – “The cache size must be between 100MB and 2,048MB.” [https://docs.vmware.com/en/VMware-Horizon-7/7.4/horizon-administration/GUID-77B22AC9-EF9F-4161-9856-88DADEE095DD.html]

3 – Search for “Common Blocks” in https://docs.vmware.com/en/VMware-Horizon-7/7.0/com.vmware.horizon-view.desktops.doc/GUID-84AB0AD4-CE68-4D40-B63A-8011586EB16E.html

4 – “View Storage Accelerator might take tens of minutes to generate or regenerate the digest files for large virtual disks (for example, a 100 GB virtual disk). As a result, the desktop might be inaccessible for longer than expected.” [https://docs.vmware.com/en/VMware-Horizon-7/7.3.3/rn/horizon-733-view-release-notes.html]

5 – “View Storage Accelerator is disabled on virtual machines that are larger than 512GB.” [https://docs.vmware.com/en/VMware-Horizon-7/7.3.3/rn/horizon-733-view-release-notes.html]