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I have installed Windows 8.1 on an iSCSI target. This target is provided by Linux LIO and is backed by a sparse file.

One of the reasons I created such an installation was thin provisioning. In other words, when I free disk space on Windows, LIO should punch holes into the file, thus free storage space on the Linux server as well.

I have checked my kernel's sources and the SCSI UNMAP command is really supported for file-backed targets. On the other hand, deleting files on Windows doesn't lower the amount of space taken by the backing file on Linux (checked with du). Actually, the backing file sometimes grows even more.

Some sources on Google say Win8 should support UNMAP/DISCARD on iSCSI, but even in Wireshark I only see ordinary read and write commands when files are being deleted. Any way to fix or troubleshoot it?

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up vote 0 down vote accepted

You need to check whether the emulate_tpu attribute is enabled for backstore (sparse file you have created and attached as LUN to iscsi target)

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You're right, it is disabled. I'll try if it helps. BTW who gave the downvote?! – LubosD Aug 19 '14 at 10:38
Don't know..glad it helped – nagato Aug 19 '14 at 10:52
It helped in the sense that Windows now sends UNMAP commands, the amount of used space (reported by du) is however still not decreasing :-( – LubosD Aug 19 '14 at 19:33
I take it back, it works. Thanks again! – LubosD Aug 19 '14 at 20:06

You may want to check that the target published that it supports UNMAP, you can read how Windows expects the target to show it in The important part is in the INQUIRY page B0h.

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