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[Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_syn

From: Kevin Wolf
Subject: [Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_sync for metadata writes"
Date: Tue, 24 Aug 2010 15:16:18 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100720 Fedora/3.0.6-1.fc12 Thunderbird/3.0.6

Am 24.08.2010 15:01, schrieb Anthony Liguori:
> On 08/24/2010 05:40 AM, Kevin Wolf wrote:
>> This reverts commit 8b3b720620a1137a1b794fc3ed64734236f94e06.
>> This fix has caused severe slowdowns on recent kernels that actually do flush
>> when they are told so. Reverting this patch hurts correctness and means that 
>> we
>> could get corrupted images in case of a host crash. This means that qcow2 
>> might
>> not be an option for some people without this fix. On the other hand, I get
>> reports that the slowdown is so massive that not reverting it would mean that
>> people can't use it either because it just takes ages to complete stuff. It
>> probably can be fixed, but not in time for 0.13.0.
>> Usually, if there's a possible tradeoff between correctness and performance, 
>> I
>> tend to choose correctness, but I'm not so sure in this case. I'm not sure 
>> with
>> reverting either, which is why I post this as an RFC only.
>> I hope to get some more comments on how to proceed here for 0.13.
> How fundamental of an issue is this?  Is this something we think we know 
> how to fix and we just don't think there's time to fix it for 0.13?

I think we can improve things basically by trying to batch metadata
writes and do them in parallel while already processing the next requests.

I'm not sure what the numbers are going to look like with something like
this in place, I need to try it. It's definitely not something that I
want to go into 0.13 at this point.

> And with this fix in place, how much confidence do we have in qcow2 with 
> respect to data integrity on power loss?

How do you measure confidence? :-)

There are power failure tests and I don't have any bugs open to that
respect. I'm not sure how intensively it's tested, though.

> We've shipped every version of QEMU since qcow2 was introduced with 
> known data corruptions.  It sucks big time.  I think it's either that 
> building an image format is a really hard problem akin to making a file 
> system and we shouldn't be in that business or that qcow2 is bad as an 
> image format which makes this all harder than it should be.

I tend to say that it's just hard to get right. Most of the problems
that were fixed in qcow2 over the last year are probably present in our
VMDK implementation as well, just to pick one example.


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