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[Qemu-devel] Re: [PATCH] Add cache=unsafe parameter to -drive

From: Alexander Graf
Subject: [Qemu-devel] Re: [PATCH] Add cache=unsafe parameter to -drive
Date: Wed, 26 May 2010 18:19:16 +0200

On 26.05.2010, at 18:17, Anthony Liguori wrote:

> On 05/26/2010 10:51 AM, Alexander Graf wrote:
>> 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 cache value to -drive that allows us to set the cache
>> policy to most aggressive, disabling flushes. We call this mode "unsafe",
>> as guest data is not guaranteed to survive host crashes anymore.
>> This patch also adds a noop function for aio, so we can do nothing in AIO
>> fashion.
>> Signed-off-by: Alexander Graf<address@hidden>
> I'm happy merging this for now, but I think it might make more sense to do 
> this as per-device flush control qdev options since those bits are actually 
> guest visible.  Christoph, does that seem reasonable?

The way it's implemented in this patch it's not guest visible. The guest still 
gets the flush capability exposed, but the actual operation returns a nop. With 
cache=unsafe qemu basically bevaves the same as a battery backed raid 


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