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

[Laurent Vivier]

Le lundi 03 décembre 2007 à 09:54 -0600, Anthony Liguori a écrit :
> 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,

Hi Anthony,

> 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.

I don't have real benchmarks. I just saw some improvements with dbench
(which is not a good benchmark, I know...)

Direct I/O can be good in some cases (because it avoids multiple copies)
and good in others (because it avoids disk access, and as you say it
doesn't wait I/O completion).

But there are at least two other good reasons to use it:

- reliability: by avoiding cache we improve probability of data are on
disk (and the ordering of I/O). And as you say, as we wait write
completion, we are sure data have been written.

- isolation: it allows to avoid to pollute host cache with guest data
(and if we have several guests, it avoids to have performance impact at
the cache level between guests).

But there is no perfect solution, it's why I think it's good thing to
let the choice to the user.

Laurent
- 
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