[Libguestfs] RFE: 'fstrim' minimum block size when sparsifying qcow2 should be 64K

[Richard W.M. Jones]

On Thu, Nov 05, 2015 at 01:10:54PM +0200, Yaniv Kaul wrote:
> As the minimum block size to discard in qcow2 is 64K, any point in
> fstrimming smaller contiguous blocks?

For virt-sparsify we just use the default that the util-linux
'fstrim -m' option uses, which (according to the fstrim manual) is 0
so it will try to discard any contiguous extent.

Unfortunately it's hard to tell what the best minimum free extent
would be.  The disk image might not be qcow2 -- if it was raw then any
4K or even 1K block could be worth freeing.

Is it a lot slower if we use a smaller minimum free extent?

Rich.

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