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Date: Fri, 14 Mar 2014 13:24:33 +0000
From: "Richard W.M. Jones" <rjones@redhat.com>
Message-ID: <20140314132433.GS1346@redhat.com>
References: <20140313214814.GC1985@redhat.com> <5322F670.50906@redhat.com>
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Subject: Re: [Qemu-devel] fstrim & upstream kernel not working
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org

On Fri, Mar 14, 2014 at 01:47:59PM +0100, Paolo Bonzini wrote:
> 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).

Not much happens in the libguestfs appliance.  There are usually only
two processes (udev + guestfsd).

> Try syncing before fstrim, not after.  In
> fact the sync before umount should not be necessary.

Yes, that works with both upstream kernel+qemu and with F21 kernel+qemu.

https://github.com/libguestfs/libguestfs/commit/d46ceea6014006ab19b6f795e2e28a7360d90b2c

Thanks,

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org