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Punching holes in files

Punching holes in files

Posted Nov 18, 2010 16:54 UTC (Thu) by chmouel (guest, #6335)
In reply to: Punching holes in files by josefwhiter
Parent article: Punching holes in files

I am not sure I catch the "sending a discard" by the guest, what does it do ?

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Punching holes in files

Posted Nov 18, 2010 18:40 UTC (Thu) by nix (subscriber, #2304) [Link]

"This bit of this file is mapping a part of a virtual filesystem known to be empty. Its contents do not matter."

Currently, the backing file still takes up space on the host's filesystem when that is done. It need not. (Possible downside: files with huge numbers of holes scattered through them had a historical propensity to get enormously fragmented, but for VM filesystem images this is probably completely unimportant, as the 'fragmentation' is going to be precisely in line with actual *files* in the contained FS.)

Punching holes in files

Posted Nov 19, 2010 16:55 UTC (Fri) by ewan (subscriber, #5533) [Link]

I suspect that this will bring us full circle to the TRIM command, and that this will actually work with the guest OS issuing a TRIM to the virtual hardware, and that being implemented on the host as 'punch hold in file', which may then also cause the host to issue a TRIM to its real underlying storage.

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