Re: [Qemu-devel] Default cache mode

[Michael Tokarev]

29.06.2011 16:32, Kevin Wolf wrote:
> Am 29.06.2011 14:23, schrieb Anthony Liguori:
>> On 06/29/2011 07:16 AM, Kevin Wolf wrote:

>>> 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.

Extremly poor performance also happens on raw devices --
be it lvm volumes or plain partitions, so that's w/o
any filesystem.  Sure, a filesystem adds quite noticeable
overhead (because of mtime updates and that sort of things),
but _just_ syncronous writes are enough to kill performance.

/mjt