On Wed, Jul 21, 2010 at 07:16:09PM +0200, Jan Kara wrote: > Hi, > > > On Wed, Jun 30, 2010 at 09:21:04AM -0400, Ric Wheeler wrote: > > > > > > The problem with not issuing a cache flush when you have dirty meta > > > data or data is that it does not have any tie to the state of the > > > volatile write cache of the target storage device. > > > > We track whether or not there is any metadata updates associated with > > the inode already; if it does, we force a journal commit, and this > > implies a barrier operation. > > > > The case we're talking about here is one where either (a) there is no > > journal, or (b) there have been no metadata updates (I'm simplifying a > > little here; in fact we track whether there have been fdatasync()- vs > > fsync()- worthy metadata updates), and so there hasn't been a journal > > commit to do the cache flush. > > > > In this case, we want to track when is the last time an fsync() has > > been issued, versus when was the last time data blocks for a > > particular inode have been pushed out to disk. > > > > To use an example I used as motivation for why we might want an > > fsync2(int fd[], int flags[], int num) syscall, consider the situation > > of: > > > > fsync(control_fd); > > fdatasync(data_fd); > > > > The first fsync() will have executed a cache flush operation. So when > > we do the fdatasync() (assuming that no metadata needs to be flushed > > out to disk), there is no need for the cache flush operation. > > > > If we had an enhanced fsync command, we would also be able to > > eliminate a second journal commit in the case where data_fd also had > > some metadata that needed to be flushed out to disk. > Current implementation already avoids journal commit because of > fdatasync(data_fd). We remeber a transaction ID when inode metadata has > last been updated and do not force a transaction commit if it is already > committed. Thus the first fsync might force a transaction commit but second > fdatasync likely won't. > We could actually improve the scheme to work for data as well. I wrote > a proof-of-concept patches (attached) and they nicely avoid second barrier > when doing: > echo "aaa" >file1; echo "aaa" >file2; fsync file2; fsync file1 > > Ted, would you be interested in something like this? Well... on my fsync-happy workloads, this seems to cut the barrier count down by about 20%, and speeds it up by about 20%. I also have a patch to ext4_sync_files that batches the fsync requests together for a further 20% decrease in barrier IOs, which makes it run another 20% faster. I'll send that one out shortly, though I've not safety-tested it at all. --D -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
