On Thu, 2010-07-29 at 15:56 -0400, Ric Wheeler wrote: > On 07/29/2010 03:49 PM, Christoph Hellwig wrote: > > On Thu, Jul 29, 2010 at 03:44:31PM -0400, Ric Wheeler wrote: > > > >> I confess that I am a bit fuzzy on FUA, but think that it means that any > >> FUA tagged IO will go down to persistent store before returning. > >> > > Exactly. > > > > > >> If so, then all order dependent IO would need to be issued in order and > >> tagged with FUA. It would not suffice to tag just the commit record as > >> FUA, or do I misunderstand what FUA does? > >> > > The commit record is ext3/4 specific terminalogy. In xfs we just have > > one type of log buffers, and we could tag that as FUA. There is very > > little other depenent I/O, but if that is present we need a pre-flush > > for it anyway. > > > > > > I assume that for ext3 it would get more complicated depending on the > journal mode. In ordered or data journal mode, we would have to write > the dependent non-journal data tagged with FUA, then the FUA tagged > transaction and finally the FUA tagged commit block. > > Not sure how FUA performs, but writing lots of small tagged writes is > probably not good for performance... That's basically everything FUA ... you might just as well switch your cache to write through and have done. This, by the way, is one area I'm hoping to have researched on SCSI (where most devices do obey the caching directives). Actually see if write through without flush barriers is faster than writeback with flush barriers. I really suspect it is. James -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
