There have been numerous questions around supported storage controllers with regards to VSAN. Those of you who have been following the various posts and watching the presentations/webinars on VSAN will have seen the requirement that pass-through mode must be supported if you wish to use a RAID Controller. The purpose of this article is to delve deeper into this requirements and explain why VMware are making that recommendation for storage controllers.
Which storage controllers can I use with Virtual SAN?
In general, we encourage beta participants to use any storage controller with Virtual SAN. However, for RAID controllers, VSAN works best if the controller supports pass-through mode.
Why are RAID controllers with pass-through support recommended?
VSAN uses magnetic disks as the persistent store for the data on the VSAN datastore and Flash as a performance acceleration layer – a read cache and write buffer – in front of the magnetic disks. All writes go to the flash layer, and all reads are first tried from the flash layer. This design obtains the lowest $/GB (using magnetic disks) and the lowest $/IOP (using Flash). While magnetic disk drives provides a low $/GB, they only support limited IOPs.
VSAN directly manages the magnetic disks, published via the pass-thru controller, in a way that the limited IOPs on the magnetic disk are used in the most optimal way. To do so, VSAN implements a proximal IO algorithm. The proximal IO algorithm is used to de-stage writes from the Flash device that is “approximately” close to each other on the magnetic disk. This design addresses the “I/O blender” situation where sequential I/O from a VM can become random when multiple VM are doing I/O to the same disk. The VSAN proximal IO algorithm turns the random I/O from the I/O blender back into sequential I/O, thus improving performance.
Will a RAID controller that does not support pass-through mode work with VSAN?
If the RAID controller does not support pass through mode, VSAN will still work. Since VSAN doesn’t control the device directly, the proximal IO algorithm built into VSAN will not function efficiently. Performance on the VSAN datastore will not be maximized in this configuration.
Besides performance, is there any functionality lost with a controller that does not support pass-through mode?
Yes. If pass-thru mode is not supported by the controller, VSAN may not function in “Auto” mode. This means that a hot-plug event will not be recognized by VSAN. In order to use the disk with VSAN, one would need to carve out a logical RAID device/LUN using the RAID controller’s utility before it is consumed by the VSAN datastore.
In addition, a hot-plug remove of the drive will not be propagated back to VSAN. Though IOs will be failed back correctly to the guest VMs, VSAN won’t detect that this is a APD/PDL condition and hence the fail-over will be handled differently by VSAN.
Are there controllers VMware is internally testing with Virtual SAN?
Yes. The following storage controllers have been tested so far:
Pass-through controllers: Dell Perc H310, HP H220i, IBM ServeRAID M1015 SAS/SATA Controller, Dell Perc H200, LSI 2008
RAID Controllers (that do not support pass-through mode): HP Smart Array P420i
Are there any known issues with storage controllers?
Currently, the AHCI SATA controller has known issues with VSAN. This manifests itself as disks/controller going into degraded mode and resulting in PDL (Permanent Device Loss). This could result in data loss and VSAN becoming unavailable. The VSAN team is actively looking at this issue.
This is a known issue only with this specific controller, and that a fix is on its way shortly. Using this controller is not recommended until the fix is available.
A live document on storage controllers for VSAN (on which this article is based) was created by our VSAN PM, Kiran Madnani, and lives here:
Check this document regularly for updates on VSAN and Storage Controllers.
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