Re: Making discard/fstrim reliable

[Paolo Bonzini <pbonzini@redhat.com>]

Il 03/04/2014 19:08, Lukáš Czerner ha scritto:
> However if we're talking about raw file system images in files in
> the host, then much better solution would be to use fsck. Ext4
> already has option -E discard which will send a discard down for
> ever free range (similarly as fstrim would do on mounted file
> system). I suspect that other fs utilities might have similar
> functionality.
>
> Of course in order for it to work you need a layer to translate
> discard requests to punch holes to the underlying file system (such
> as loop device for example). But I think that if there is enough
> interest we might do this directly from e2fsck when we notice that
> we're running on the file rather than block device.

The e2fsck can also be done from within libguestfs easily.  libguestfs 
runs within a VM so QEMU would handle the translation to hole-punching.

But fstrim is much faster than e2fsck.

 From Richard's description, what seems to happen is that ext4 FITRIM 
scans the filesystem and prepares the discard requests; but then it 
sends them down to the filesystem after the ioctl has finished.  Does 
that make any sense?  And would that be considered a bug?

Paolo
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