Avoiding SCSI reservations by Spanning

[MartinWi]

Hello Community,

After taking a look at the "Scalable Storage Performance" document a quick thought popped into mind. According to the document the first LUN in a spanned VMFS is where the SCSI reservations will take place, regardless of where the actual VM thats being worked on is placed. So, the idea i this:, can you more or less avoid the negative impacts of SCSI reservations by creating a tiny LUN first, and then adding larger LUNS by spanning so that effectively all VM's are on the larger LUNs and thus not affected by SCSI reservations?

[glynnd1 · Accepted solution]

Your though does not resolve the SCSI reservation issue. Currently I have the SCSI reservation, and you need it. However, until I am finished, you must wait.

It doesn't matter if the reservation is against all the LUNs that make up an spanned VMFS volume, or just against the first LUN, we are both fighting to own the same reservation.

[mike_laspina]

Hi,

Unfortunately that's not the case. Within any VMFS store you have shared elements that are used to perform clustered operation and state control. All of those operation occur on the primary VMFS LUN and thus you will actually compound the reservation issue rather ran reduce it.

Regards,

Mike

vExpert 2009

[MartinWi]

Ok, I see. That means that the VM's are affected even though they're on a different LUN, because they're part of the same volume. Makes sense I guess.

[glynnd1]

They are impacted if they need a SCSI reservation and their host cannot get one. The vast majority of the time a VM don't need a SCSI reservation, so this isn't a issue.

VMware improved the SCSI reservation suitation with the release of vSphere with some streamlining, and I'm sure in the future we can see further improvements.