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Date: Fri, 14 Mar 2014 13:47:59 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] fstrim & upstream kernel not working
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To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: qemu-devel@nongnu.org

Il 14/03/2014 13:42, Richard W.M. Jones ha scritto:
> I worked around this in any case by rearranging the test [2]:
>
> Doing:
>
>   rm /a_big_file
>   fstrim /
>   sync
>   umount /
>   [shut down qemu]
>
> would only trim 64 KB on the host.
>
> Doing:
>
>   rm /a_big_file
>   umount /                         # added
>   mount -o nodiscard /dev/sda /    # added
>   fstrim /
>   sync
>   umount /
>   [shut down qemu]
>
> would trim the expected amount (around 10 MB).
>
> I've no idea why this is (looks like an ext4/kernel bug to me), but in
> any case the tests now use the second method[2].

Could be a race condition (something going on in the background between 
rm and fstrim).  Try syncing before fstrim, not after.  In fact the sync 
before umount should not be necessary.

Paolo