Re: Linux 2.6.29

Ric Wheeler
2009-03-27T15:07:25Z

Christoph Hellwig wrote:
> On Fri, Mar 27, 2009 at 10:13:33AM -0400, Theodore Tso wrote:
>
>> On Fri, Mar 27, 2009 at 08:57:23AM +0100, Jens Axboe wrote:
>>
>>> Here's a simple patch that does that. Not even tested, it compiles. Note
>>> that file systems that currently do blkdev_issue_flush() in their
>>> ->sync() should then get it removed.
>>>
>>>
>> That's going to be a mess. Ext3 implements an fsync() by requesting a
>> journal commit, and then waiting for the commit to have taken place.
>> The commit happens in another thread, kjournald. Knowing when it's OK
>> not to do a blkdev_issue_flush() because the commit was triggered by
>> an fsync() is going to be really messy. Could we at least have a flag
>> in struct super which says, "We'll handle the flush correctly, please
>> don't try to do it for us?"
>>
>
> Doing it in vfs_fsync also is completely wrong layering. If people want
> it for simple filesystems add it to file_fsync instead of messing up
> the generic helper. Removing well meaning but ill behaved policy from
> the generic path has been costing me far too much time lately.
>
> And please add a tuneable for the flush. Preferable a generic one at
> the block device layer instead of the current mess where every
> filesystem has a slightly different option for barrier usage.
>

I agree that we need to be careful not to put extra device flushes if
the file system handles this properly. They can be quite expensive (say
10-20ms on a busy s-ata disk).

I have also seen some SSD devices have performance that drops into the
toilet when you start flushing their volatile caches.

ric