Re: [Qemu-devel] Caching modes

[Anthony Liguori <anthony@codemonkey.ws>]

On 09/20/2010 01:37 PM, Blue Swirl wrote:
>
> It would be nice to have additional mode, like cache=always, where
> even flushes MAY be ignored. This would max out the performance.
>    

That's cache=unsafe and we have it.  I ignored it for the purposes of 
this discussion.

>> Guest disk cache.
>>
>> For all devices that support it, the exposed cache attribute should be
>> independent of the host caching mode.  Here are correct usages of disk
>> caching mode:
>>
>> Writethrough disk cache; cache=none|writethrough if the disk cache is set to
>> writethrough or the disk is considered "enterprise class" and has a battery
>> backup.  cache=writeback IFF the host is backed by an UPS.
>>      
> The "enterprise class" disks, battery backups and UPS devices are not
> consumer equipment. Wouldn't this mean that any private QEMU user
> would need to use cache=none?
>    

No, cache=writethrough and cache=none should be equivalent from a data 
integrity/data loss perspective.  Using cache=writeback without 
enterprise storage is risky but practically speaking, most consumer 
storage is not battery backed and uses writeback caching anyway so there 
is already risk.

> As an example, what is the correct usage for laptop user, considering
> that there is a battery, but it can also drain and the drainage is
> dependent on flush frequency?
>    

Minus cache=unsafe, you'll never get data corruption.  The only 
consideration is how much data loss can occur from the last time there 
was a flush.  Well behaved applications always flush important data to 
avoid loss of anything important but practically speaking, the world 
isn't full of behaved applications.

The only difference between cache=writeback and a normal disk's 
writeback cache is that cache=writeback can be a very, very large cache 
that isn't frequently flushed.  So the amount of data loss can be much 
higher than expected.

For most laptop users, cache=none or cache=writethrough is appropriate.  
For a developer, cache=writeback probably is reasonable.

Regards,

Anthony Liguori

Regards,

Anthony Liguori