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Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe |
Date: |
Mon, 24 Oct 2011 10:05:25 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:7.0) Gecko/20110927 Thunderbird/7.0 |
Am 23.10.2011 16:33, schrieb Paolo Bonzini:
> On 10/22/2011 05:07 PM, Alexander Graf wrote:
>>
>> On 21.10.2011, at 11:44, Paolo Bonzini wrote:
>>
>>> On 10/21/2011 07:08 PM, Kevin Wolf wrote:
>>>> Avi complained that not even writing out qcow2's cache on
>>>> bdrv_flush() made cache=unsafe too unsafe to be useful. He's got
>>>> a point.
>>>
>>> Why? cache=unsafe is explicitly allowing to s/data/manure/ on
>>> crash.
>>
>> Exactly, but not on kill. By not flushing internal caches you're
>> almost guaranteed to get an inconsistent qcow2 image.
>
> This should be covered already by termsig_handler. bdrv_close_all
> closes all block devices, and qcow2_close does flush the caches.
>
> SIGKILL doesn't give any guarantee of course but it does not in general,
> even without cache=unsafe; you might get a SIGKILL "a moment before" a
> bdrv_flush even without cache=unsafe, and get unclean qcow2 metadata.
Unclean yes, in the sense that you may get cluster leaks. If getting
SIGKILL "a moment before" the flush led to real corruption however,
cache=none would be broken as well.
>> By not flushing internal caches you're almost guaranteed to get an
>> inconsistent qcow2 image.
>
> Of course the inconsistencies with cache=unsafe will be massive if you
> don't have a clean exit, but that's expected. If in some cases you want
> a clean exit, but right now you don't, the place to fix those cases
> doesn't seem to be the block layer, but the main loop.
I don't think there's much the main loop can do against SIGKILL,
segfaults or abort().
> Also,
>
> 1) why should cache=unsafe differentiate an OS that sends a flush from
> one that doesn't (e.g. MS-DOS), from the point of view of image metadata?
>
> 2) why should the guest actually send a flush if cache=unsafe? Currently
>
> if (flags & BDRV_O_CACHE_WB)
> bs->enable_write_cache = 1;
>
> covers cache=unsafe. However, in the end write cache enable means "do I
> need to flush data", and the answer is "no" when cache=unsafe, because
> the flushes are useless and guests are free to reorder requests.
>
> <shot-in-the-dark>Perhaps what you want is to make qcow2 caches
> writethrough in cache=unsafe mode, so that at least a try is made to
> write the metadata</shot-in-the-dark> (even though the underlying raw
> protocol won't flush it)? I'm not sure that is particularly useful, but
> maybe it can help me understanding the benefit of this change.
Yes, this is the intention. It's about flushing metadata, not guest
data. The semantics that I think cache=unsafe should have is that after
a bdrv_flush() we have flushed all caches in qemu (so that the image
survives a qemu crash), but we don't care about flushing the host page
cache.
Kevin
- [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Kevin Wolf, 2011/10/21
- [Qemu-devel] [PATCH 2/2] block: Handle cache=unsafe only in raw-posix/win32, Kevin Wolf, 2011/10/21
- [Qemu-devel] [PATCH 1/2] raw-posix: Convert to bdrv_co_flush, Kevin Wolf, 2011/10/21
- Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Paolo Bonzini, 2011/10/21
- Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Kevin Wolf, 2011/10/24
- Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Paolo Bonzini, 2011/10/24
- Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Kevin Wolf, 2011/10/24
- Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Paolo Bonzini, 2011/10/24
- Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Kevin Wolf, 2011/10/24
- Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Paolo Bonzini, 2011/10/24
- Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Kevin Wolf, 2011/10/24
- Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Paolo Bonzini, 2011/10/24
- Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Kevin Wolf, 2011/10/24
- Re: [Qemu-devel] [PATCH 0/2] block: Write out internal caches even with cache=unsafe, Paolo Bonzini, 2011/10/24