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I’m talking about the case where the storage device uses thin-provisioning internally to allow for better utilization. In this particular case the vendor array uses thin-provisioning, and I don’t have an option to turn it off. What I see is something like this:</div>
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- Create a 10 TB file systems</div>
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- Fill it with 4 TB of data</div>
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- Delete the data (from the host)</div>
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- Storage array still reports 4 TB of usage while the host sees 0</div>
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This evidently goes back to the use of the SCSI "UNMAP” call that you can add as a mount option in RedHat Linux (example)</div>
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<span style="color: rgb(51, 51, 51); font-family: 'Courier New';">mount </span><span style="font-family: 'Courier New';"><font color="#ff0000">-o discard </font></span><span style="color: rgb(51, 51, 51); font-family: 'Courier New';">LABEL=DemoVol /files/</span></div>
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Google "redhat linux scsi unmap” and you’ll see references to this.</div>
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<div><font face="Helvetica,sans-serif">However, GPFS doesn’t support this (see my previous reference) and as a result the array doesn’t know the block is no longer in use. This doesn’t mean GPFS can’t re-use it, it just means the array thinks there is more
in use than there really is. I don’t know if this is common with thinly-provisioned arrays in general or specific to this vendor. But the fact that IBM call it out (“s</font><font face="Helvetica,sans-serif">ince at present GPFS does not communicate block
deallocation events to the block device layer”)<font color="#ff0000"> </font></font><font face="Helvetica,sans-serif">means that they are aware of this behavior in some arrays – perhaps their own as well.</font></div>
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<font face="Helvetica">Bob Oesterlin<br>
Sr Storage Engineer, Nuance HPC Grid<br>
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<span style="font-weight:bold">From: </span><<a href="mailto:gpfsug-discuss-bounces@spectrumscale.org">gpfsug-discuss-bounces@spectrumscale.org</a>> on behalf of Jonathan Buzzard <<a href="mailto:jonathan@buzzard.me.uk">jonathan@buzzard.me.uk</a>><br>
<span style="font-weight:bold">Reply-To: </span>gpfsug main discussion list <<a href="mailto:gpfsug-discuss@spectrumscale.org">gpfsug-discuss@spectrumscale.org</a>><br>
<span style="font-weight:bold">Date: </span>Tuesday, December 22, 2015 at 5:46 PM<br>
<span style="font-weight:bold">To: </span>"<a href="mailto:gpfsug-discuss@spectrumscale.org">gpfsug-discuss@spectrumscale.org</a>" <<a href="mailto:gpfsug-discuss@spectrumscale.org">gpfsug-discuss@spectrumscale.org</a>><br>
<span style="font-weight:bold">Subject: </span>Re: [gpfsug-discuss] Experiences with thinly-provisioned volumes?<br>
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<span style="color: rgb(0, 0, 0); font-family: -webkit-standard; font-size: medium; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; display: inline !important; float: none;">What
is the usage case of using thin provisioning on a GPFS file system?</span></span>
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