First of all, why are you people sending TWO copies to the XFS mailing list? (to both linux-xfs@xxxxxxxxxxx and xfs@xxxxxxxxxxx). >>> At the moment it appears to me that disabling write cache >>> may often give more performance than using barriers. And >>> this doesn't match my expectation of write barriers as a >>> feature that enhances performance. >> Why do you have that expectation? I've never seen barriers >> advertised as enhancing performance. :) This entire discussion is based on the usual misleading and pointless avoidance of the substance, in particular because of stupid, shallow diregard for the particular nature of the "benchmark" used. Barriers can be used to create atomic storage transaction for metadata or data. For data, they mean that 'fsync' does what is expected to do. It is up to the application to issue 'fsync' as often or as rarely as appropriate. For metadata, it is the file system code itself that uses barriers to do something like 'fsync' for metadata updates, and enforce POSIX or whatever guarantees. The "benchmark" used involves 290MB of data in around 26k files and directories, that is the average inode size is around 11KB. That means that an inode is created and flushed to disk every 11KB written; a metadata write barrier happens every 11KB. A synchronization every 11KB is a very high rate, and it will (unless the disk host adapter or the disk controller are clever mor have battery backed memory for queues) involve a lot of waiting for the barrier to complete, and presumably break the smooth flow of data to the disk with pauses. Also whether or not the host adapter or the conroller write cache are disabled, 290MB will fit inside most recent hosts' RAM entirely, and even adding 'sync' at the end will not help that much as to helping with a meaningful comparison. > My initial thoughts were that write barriers would enhance > performance, in that, you could have write cache on. Well, that all depends on whether the write caches (in the host adapter or the controller) are persistent and how frequently barriers are issued. If the write caches are not persistent (at least for a while), the hard disk controller or the host adapter cannot have more than one barrier completion request in flight at a time, and if a barrier completion is requested every 11KB that will be pretty constraining. Barriers are much more useful when the host adapter or the disk controller can cache multiple transactions and then execute them in the order in which barriers have been issued, so that the host can pipeline transactions down to the last stage in the chain, instead of operating the last stages synchronously or semi-synchronously. But talking about barriers in the context of metadata, and for a "benchmark" which has a metadata barrier every 11KB, and without knowing whether the storage subsystem can queue multiple barrier operations seems to be pretty crass and meangingless, if not misleading. A waste of time at best. > So its really more of an expectation that wc+barriers on, > performs better than wc+barriers off :) This is of course a misstatement: perhaps you intended to write that ''wc on+barriers on'' would perform better than ''wc off + barriers off'. As to this apparent anomaly, I am only mildly surprised, as there are plenty of similar anomalies (why ever should have a very large block device readahead to get decent performance from MD block devices?), due to poorly ill conceived schemes in all sorts of stages of the storage chain, from the sometimes comically misguided misdesigns in the Linux block cache or elevators or storage drivers, to the often even worse "optimizations" embedded in the firmware of host adapters and hard disk controllers. Consider for example (and also as a hint towards less futile and meaningless "benchmarks") the 'no-fsync' option of 'star', the reasons for its existence and for the Linux related advice: http://gd.tuwien.ac.at/utils/schilling/man/star.html «-no-fsync Do not call fsync(2) for each file that has been extracted from the archive. Using -no-fsync may speed up extraction on operating systems with slow file I/O (such as Linux), but includes the risk that star may not be able to detect extraction problems that occur after the call to close(2).» Now ask yourself if you know whether GNU tar does 'fsync' or not (a rather interesting detail, and the reasons why may also be interesting...). -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
