Re: [Qemu-devel] [PATCH 2/2 v2] Direct IDE I/O

[Anthony Liguori <anthony@codemonkey.ws>]

Laurent Vivier wrote:
> Le lundi 03 décembre 2007 à 11:23 +0100, Fabrice Bellard a écrit :
>   
>> Laurent Vivier wrote:
>>     
>>> This patch enhances the "-drive ,cache=off" mode with IDE drive emulation
>>> by removing the buffer used in the IDE emulation.
>>> ---
>>>  block.c     |   10 +++
>>>  block.h     |    2 
>>>  block_int.h |    1 
>>>  cpu-all.h   |    1 
>>>  exec.c      |   19 ++++++
>>>  hw/ide.c    |  176 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
>>>  vl.c        |    1 
>>>  7 files changed, 204 insertions(+), 6 deletions(-)
>>>       
>> What's the use of keeping the buffered case ?
>>     
>
> Well, I don't like to remove code written by others...
> and I don't want to break something.
>
> But if you think I should remove the buffered case, I can.
>
> BTW, do you think I should enable "cache=off" by default ?
> Or even remove the option from the command line and always use
> O_DIRECT ?
>   

Hi Laurent,

Have you done any performance testing?  Buffered IO should absolutely 
beat direct IO simply because buffered IO allows writes to complete 
before they actually hit disk.  I've observed this myself.  Plus the 
host typically has a much larger page cache then the guest so the second 
level of caching helps an awful lot.

Regards,

Anthony Liguori

> Regards,
> Laurent
>