Making discard/fstrim reliable

["Richard W.M. Jones" <rjones@redhat.com>]

virt-sparsify is a tool for trimming free space in virtual disk
images.  The new implementation uses vfs/kernel/qemu discard support.
Essentially it does:

  for each filesystem:
    mount -o discard $fs /mnt
    sync
    fstrim /mnt
    umount /mnt
  sync
  # qemu is killed after sync returns

Although typing these commands by hand works fine, when you run them
from a program the fstrim doesn't happen all the way down the stack
reliably.  Mostly it works, but sometimes it only trims some space
from the host file.

It appears that when the host is slow / under load, the problem
happens more frequently.  Also it may happen more frequently on i686
than on x86-64 (possibly also due to speed of host).

The question is: What can I do to make sure the trim happens reliably,
all the way down the stack, before qemu is killed?

I am testing this using the latest upstream kernel & qemu.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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