Messages in this thread | | | Date | Fri, 27 Mar 2009 11:03:01 -0400 | From | Ric Wheeler <> | Subject | Re: Linux 2.6.29 |
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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
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