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Date: Mon, 03 Jun 2013 08:39:16 +0200
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Subject: Re: [Qemu-devel] fstrim and cache=none
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To: Dusty Mabe <dustymabe@gmail.com>
Cc: qemu-devel@nongnu.org

Il 02/06/2013 22:21, Dusty Mabe ha scritto:
> Hi,
> 
> Is it valid to have cache=none and discard=unmap together? I notice
> that when I have cache=none the fstrim operations inside of my guests
> gives me an error

What error?  Can you add an extra disk and try

dd if=/dev/urandom of=/dev/sdc bs=1M count=32
sg_unmap -l0 -n64 /dev/sdc

from within the guest?

> and i get this from the sys filesystem:
> 
> [root@guest ~]# cat /sys/block/sdb/queue/discard*
> 4096
> 0
> 0

This is expected.  It won't change between discard=unmap and
discard=ignore, too.  (Tip: Use "grep . /sys/block/sdb/queue/discard*"
so that it shows the filename).

> 
> 
> This wasn't very obvious to me to begin with and just wanted to see if
> this is a bug or as designed.

It should be.  It may be a Linux bug, too.

Paolo