Re: [Qemu-devel] Default cache mode

[Kevin Wolf <kwolf@redhat.com>]

Am 29.06.2011 14:23, schrieb Anthony Liguori:
> On 06/29/2011 07:16 AM, Kevin Wolf wrote:
>> Am 29.06.2011 14:06, schrieb Anthony Liguori:
>>> On 06/29/2011 06:59 AM, Kevin Wolf wrote:
>>>> Hi,
>>>>
>>>> I think we have touched this topic before during some IRC discussions or
>>>> somewhere deep in a mailing list thread, but I think it hasn't been
>>>> discussed on the list.
>>>>
>>>> Our default cache mode of cache=writethrough is extremely conservative
>>>> and provides absolute safety at the cost of performance,
>>>
>>> But for the most part, we track bare metal fairly well in terms of block
>>> performance, no?
>>>
>>> Or are you really referring to qcow2 as a specific example?  In the
>>> past, we used a different default caching mode for qcow2.  I think that
>>> could be done again if there was a compelling reason.
>>
>> No, people are also complaining about bad performance with raw. Which
>> isn't really surprising when you do a flush after each single write
>> request. O_SYNC is really much more than is needed in the average case.
> 
> Which file system on the host?

Honestly, I don't know. Usually the problem is resolved with setting a
different cache option, so nobody bothers to ask for details. I'd guess
that it's ext4 in most cases.

> At any rate, I'm a big fan of making wce tunable in the guest and then I 
> think setting wce=1 is quite reasonable to do by default.

Okay, I think that's a fair requirement. So let's plan to implement this
in 0.16.

Christoph, it looks like we're back to your WCE patches then. Do you
still work on them?

Kevin