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

From: Kevin Wolf
Subject: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive
Date: Wed, 12 May 2010 17:36:45 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4

Am 12.05.2010 17:05, schrieb Alexander Graf:
> 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.

By doing the check in bdrv_flush/bdrv_aio_flush before this function
pointer is even called.


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