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Heads up: Issues with large SCSI UNMAP requests with vSphere 5/5.1

ndyer39
Honored Contributor

Heads up: Issues with large SCSI UNMAP requests with vSphere 5/5.1

Came across this new blog post from Cormac Hogan (@vmwarestorage) who has written about a problem that EMC have discovered around SCSI UNMAP resize requests in vSphere 5 or 5.1.

Essentially there is a 2TB file limit in the latest VMware OS', and thus if you try and send a SCSI UNMAP command it will create a balloon file to enable it to move blocks and shrink the space. This balloon file cannot be bigger than the 2TB limit.

When using SCSI UNMAP you tell it to resize a percentage of the original volume size - so rather than telling the command to UNMAP a large percentage, it's recommended to perform a series of requests of smaller resize amounts to ensure you don't hit the 2TB ceiling.

Original blog post is available here: http://cormachogan.com/2013/05/14/heads-up-unmap-considerations-when-reclaiming-more-than-2tb/

Incidentally, i'd be interested to hear your views on SCSI UNMAP. Do you use it? If so, how often do you find you use it, and how much space do you reclaim (on average?)

2 REPLIES
Not applicable

Re: Heads up: Issues with large SCSI UNMAP requests with vSphere 5/5.1

I was in a bit of a panic a few months ago because the available space on our CS-210 was decreasing more as I tried to do things to free up space.  Didn't realize I needed to run the UNMAP cmd.  After support calls to Nimble and VMware for some guidance, when I was able to run it, I recovered close to 50% of the total SAN space.  I just noticed the other day that our space has been decreasing again, so I guess it is time to run UNMAP again.

J Benway
Advisor

Re: Heads up: Issues with large SCSI UNMAP requests with vSphere 5/5.1

Does hyperv have the same issue with the need to run the UNMAP cmd?

I'm new to ISCI thanks,jb