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[Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive

From: Alexander Graf
Subject: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive
Date: Wed, 12 May 2010 17:05:08 +0200
User-agent: Thunderbird (X11/20090817)

Kevin Wolf wrote:
> Am 10.05.2010 23:51, schrieb Alexander Graf:
>> Usually the guest can tell the host to flush data to disk. In some cases we
>> don't want to flush though, but try to keep everything in cache.
>> So let's add a new parameter to -drive that allows us to set the flushing
>> behavior to "on" or "off", defaulting to enabling the guest to flush.
>> Signed-off-by: Alexander Graf <address@hidden>
> What about another cache=... value instead of adding more options? I'm
> quite sure you'll only ever need this with writeback caching. So we
> could have cache=none|writethrough|writeback|wb-noflush or something
> like that.

Yes, cache=volatile seems reasonable. Or cache=unsafe.

>> ---
>>  block/raw-posix.c |   13 +++++++++++++
> This is obviously wrong. If you want to introduce new behaviour to the
> block layer, you must do it consistently and not just for one block
> driver. So these changes should be made to the generic functions in
> block.c instead.

How so? The callback functions are called using bdrv->drv->xxx. If I
modify that pointer, I end up affecting all other virtual disks as well.


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