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September 6th, 2017 01:00

Possible impact of Scsi unmap operation with XIO

IHAC who is planning to install new XIO X2 to connect ESXi hosts via FC and place data store volumes.

They're now discussing their daily operational flow and looking for information which helps to identify possible system impact (from ESXi and XIO viewpoint) of scsi unmap operation invoked from ESXi hosts by issuing "esxcli storage vmfs unmap" to cleanup some blocks previously used for unnecessary vmdks. Just for reference, they anticipate about roughly 1TB sized volumes in each.

Any information would be highly appreciated like experimental result in lab or technical comments or advise etc... If it's not encouraged to do this kind of operation, they would like to propose their end user to do it in offline or maintenance windows.

Thanks in advise.

17 Posts

September 6th, 2017 07:00

So I am a customer, but I can tell you that my local team was all for it. In ESXi versions earlier than 6.5 the unmap operations don't happen automatically so they recommended we do it in order to free up unused blocks on the XtremIO. But that's partially because we were doing things that resulted in there being a lot of unused blocks regularly. If you never delete things it won't be as important for your environment. In our case, we do a lot of VM clones and deletes, which is what results in our unused blocks.

If you do this, I highly recommend using the "--reclaim-unit=200" option and set it to a number higher than the default of  200 to have the unmaps go much faster. We use around 20000 here with good success, but the right setting will vary in your environment. For the record, we don't notice an end user impact when we do this, but it's clear a lot of activity is going on, so again, your environment may notice it more than we do.

The good news is that when you upgrade to ESXi 6.5 and use the settings recommended in the host attachment guide for 6.5 you shouldn't have to manually do this anymore.

24 Posts

September 13th, 2017 22:00

Thank you cprivite, it really helps. As far as i went over available manuals and WPs, it seems no document is mentioning how to size or evaluate performance impact by scsi unmap operation. I totally agree it all depends on Customer's environment and intended operation.

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