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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 15:31:42 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 05.05.2015 um 15:07 hat Paolo Bonzini geschrieben:
> On 05/05/2015 15:03, Kevin Wolf wrote:
> > > 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.
> I'm not seeing corruption; it's a crash in drive-mirror.

Ah, yes, sorry. I was confused.

> The question is how to treat discard:
> 1) mark sectors as dirty, discard on the destination as well (similar to
> Fam's just-posted patches, but they do not discard on the destination)
> 2) keep marking sectors as not dirty, change util/hbitmap.c to avoid the
> assertion failure.  This loses the discard on the destination but no
> need to change block/mirror.c
> Both are valid approaches.

Which kind of suggests that it should be an option. But if I had to
choose one, I think 1) is nicer because it results in an exact copy.


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