> [ ... ] I also tried different Linux IO elevators, as you > suggested in your other response, without any measurable > effect. [ ... ] That's probably because of the RAID6 host adapter being uncooperative, but I wondered whether this might apply in some form: http://xfs.org/XFS_FAQ#Q:_I_want_to_tune_my_XFS_filesystems_for_.3Csomething.3E «As of kernel 3.2.12, the default i/o scheduler, CFQ, will defeat much of the parallelization in XFS.» BTW earlier 'cfq' versions have been reported to have huge problems with workloads involving writes and reads, and only 'deadline' (which is quite unsuitable for some workloads) seems to be fairly reliable. http://www.webhostingtalk.com/showthread.php?t=727173 «Anyway, we have found HUGE problems with CFQ in many different scenarios and many different hardware setups. If it was only an issue with our configuration I would have foregone posting this message and simply informed those kernel developers responsible for the fix. Two scenarios where CFQ has a severe problem - When you are running a single block device (1 drive, or a raid 1 scenario) under certain circumstances where heavy sustained writes are occurring the CFQ scheduler will behave very strangely. It will begin to give all access to reads and limit all writes to the point of allowing only 0-2 I/O write operations being allowed per second vs 100-180 read operations per second. This condition will persist indefinitely until the sustained write process completes. This is VERY bad for a shared environment where you need reads and writes to complete regardless of increased reads or writes. This behavior goes beyond what CFQ says it is supposed to do in this situation - meaning this is a bug, and a serious one at that. We can reproduce this EVERY TIME. The second scenario occurs when you have two or more block devices, either single drives, or any type of raid array including raid 0,1,0+1,1+0,5 and 6. (We never tested 3,4 who uses raid 3 or 4 anymore anyway?!!). This case is almost exactly opposite of what happens with only one block device. In this case if one of more of the drives is blocked with heavy writes for a sustained period of time CFQ will block reads from the other devices or severely limit the reads until the writes have completed. We can also reproduce this behavior with test software we have written on a 100% consistent basis.» _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
