On Thu, 2010-07-29 at 19:04 -0400, Ted Ts'o wrote: > On Thu, Jul 29, 2010 at 04:30:54PM -0600, Andreas Dilger wrote: > > Like James wrote, this is basically everything FUA. It is OK for > > ordered mode to allow the device to aggregate the normal filesystem > > and journal IO, but when the commit block is written it should flush > > all of the previously written data to disk. This still allows > > request re-ordering and merging inside the device, but orders the > > data vs. the commit block. Having the proposed "flush ranges" > > interface to the disk would be ideal, since there would be no wasted > > time flushing data that does not need it (i.e. other partitions). > > My understanding is that "everything FUA" can be a performance > disaster. That's because it bypasses the track buffer, and things get > written directly to disk. So there is no possibility to reorder > buffers so that they get written in one disk rotation. Depending on > the disk, it might even be that if you send N sequential sectors all > tagged with FUA, it could be slower than sending the N sectors > followed by a cache flush or SYNCHRONIZE_CACHE command. I think we're getting into disk differences here. This certainly isn't correct for SCSI disks. The standard enterprise configuration for a SCSI disk is actually cache set to write through ... so FUA is a nop. Even for Write Back cache SCSI devices, FUA is just a wait until I/O is on media, which is pretty much equivalent to the write through case for the given cache lines. I can see the problems you describe possibly affecting ATA devices with less sophisticated caches ... but, realistically, SATA and SAS devices come from virtually the same manufacturing process ... I'd be really surprised if they didn't share caching technologies. > It may be worth doing some experiments to see how big N is for various > disks, but I'm pretty sure that FUA will probably turn out to not be > such a great idea for ext3/ext4. I think we should definitely run the benchmarks. 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
