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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] Fwd: qemu drive mirror assert fault
Date: Tue, 05 May 2015 15:07:31 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

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.

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.


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