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PS4000 Volume space not releasing?
Hi All, couple of questions. Wondering why I see very different space available numbers in the EQL web gui vs. vsphere client? In the EQL gui, I'm showing 330GB free, but in vsphere client, I'm showing 674GB free. What also appears to be occurring is that when I use up space on my EQL volume and then later free up that space, it does not appear that EQL is releasing the space. e.g., I svmotion a large vm to volume 1 and Dell GUI reports the additional space used. I then move the same system to volume 2, but Dell GUI still shows the same space used on volume 1?
Anonymous
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October 5th, 2012 13:00
Yes, that is exactly what it means, but that's fine. The array doesn't tell the OS, it's "full". The OS, will reuse blocks as needed. It's the OS that knows what blocks are available. It does that through its allocation table. The array simply responds to requests from the server. In SCSI when writing you typically see the SCSI command "WRITE10". That has the block number to save it to and the data to put there. The array just does as it's told. It doesn't care that it's already been written to.
With the new UNMAP SCSI command, the OS can now pass that info along to the storage device.
It basically says UNMAP/FREE block X
For VMware, It's part of the VAAI suite introduced in ESX v4.1. Here's and FAQ on it.
kb.vmware.com/.../search.do
Anonymous
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October 5th, 2012 09:00
Q1: That's because the array is block storage, we can only record the data actually written to us. When ESX creates a 50GB VMDK, it does not write out 50GB worth of data. So the filesystem is the final authority here.
Q2: Freeing up space. Under normal SCSI operations, a delete is a WRITE to a file allocation table maintained by the host OS. The array is not told that those blocks are now free. So there's no way for the array to release those blocks back into the free space pool. There is a new SCSI command called "UNMAP". Vmware calls it "Re-Thinning". This command tells the storage to release the specified blocks.
Only ESXi v5.0 Update 1, and ESXi v5.1 support this feature. However, it is NOT automatic. You have to run a command to tell ESX to reclaim space.
Here's a link on how to do that.
kb.vmware.com/.../search.do
You need to be running EQL FW v6.0.1, or better, in order to do this. Also replicated volumes can't be reclaimed at this time.
Other OS's will do this reclaim automatically. Windows 8, Server 2012, Linux EXT4 filesystems are some examples. Note: EXT4 you need to add 'discard' to the mount options in /etc/fstab to get this to work.
Regards,
dkraut
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October 5th, 2012 10:00
Don, thanks for quick response. However, wouldn't that mean that if I continue to svmotion things in and out of volume 1, eventually the SAN will think it's full, when in reality it's not?
Stephan-thevalley
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October 9th, 2012 09:00
Any change UNMAP SCSI will be supported on Windows 2008 R2 ?
Anonymous
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October 9th, 2012 11:00
That would be up to Microsoft. I suspect it would be a significant effort on their part to do so.