[Qemu-devel] fstrim & upstream kernel not working

[Richard W.M. Jones]

I got fstrim happily working in Fedora 20, but it's not working with
the upstream kernel.  The message is:

  fstrim -v /sysroot/
  [   45.541339] sda: WRITE SAME failed. Manually zeroing.
  /sysroot/: 47.2 MiB (49466368 bytes) trimmed

While this isn't technically an error, it of course doesn't trim
anything.  In fact the host disk grows after the fstrim.

A couple of questions:

- Is there any reason why virtio-scsi doesn't emulate WRITE SAME?  It
seems pretty simple, and upstream kernels issue WRITE SAME when they
want to zero large areas of disk.

- Can you see where ext4 issues the zeroout/write same call?  AFAICT
it is still issuing discards, but these are getting turned into
zeroout/write same by some sort of block layer magic that I can't
quite follow.

kernel: 3.14.0-0.rc6.git2.1.fc21
qemu: 1.7.0-5.fc21.x86_64

Rich.

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