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Re: [Qemu-devel] Fwd: qemu drive mirror assert fault

From: Kevin Wolf
Subject: Re: [Qemu-devel] Fwd: qemu drive mirror assert fault
Date: Tue, 5 May 2015 15:03:10 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 05.05.2015 um 13:49 hat Paolo Bonzini geschrieben:
> On 05/05/2015 13:48, Kevin Wolf wrote:
> > It depends. In its basic form, bdrv_discard() just means "I don't care
> > about the data any more". Then clearing the dirty bitmap is correct.
> > 
> > The content is only important if the caller used discard to write zeros
> > because can_write_zeroes_with_unmap = true. Do we have any such callers
> > apart from qemu-img convert?
> Yes, the SCSI command WRITE SAME with UNMAP = 1 (not coincidentially :))
> calls discard too.  Who knows what the guest used it for...
> However, write zeroes doesn't go through bdrv_co_discard, does it?

Initially I expected that it does, but when I checked, it turned out
that it uses a different path. The only thing I found that really uses
discard is the call in qemu-img that I mentioned. But that can't be the
cause of the corruption you're debugging.


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