On Tue, Jan 26 2016, Chien Lee wrote: > Hello, > > Recently we find a bug about this patch (commit No. is > ac8fa4196d205ac8fff3f8932bddbad4f16e4110 ). > > We know that this patch committed after Linux kernel 4.1.x is intended > to allowing resync to go faster when there is competing IO. However, > we find the performance of random read on syncing Raid6 will come up > with a huge drop in this case. The following is our testing detail. > > The OS what we choose in our test is CentOS Linux release 7.1.1503 > (Core) and the kernel image will be replaced for testing. In our > testing result, the 4K random read performance on syncing raid6 in > Kernel 4.2.8 is much lower than in Kernel 3.19.8. In order to find out > the root cause, we try to rollback this patch in Kernel 4.2.8, and we > find the 4K random read performance on syncing Raid6 will be improved > and go back to as what it should be in Kernel 3.19.8. > > Nevertheless, it seems that it will not affect some other read/write > patterns. In our testing result, the 1M sequential read/write, 4K > random write performance in Kernel 4.2.8 is performed almost the same > as in Kernel 3.19.8. > > It seems that although this patch increases the resync speed, the > logic of !is_mddev_idle() cause the sync request wait too short and > reduce the chance for raid5d to handle the random read I/O. This has been raised before. Can you please try the patch at the end of http://permalink.gmane.org/gmane.linux.raid/51002 and let me know if it makes any difference. If it isn't sufficient I will explore further. Thanks, NeilBrown > > > Following is our test environment and some testing results: > > > OS: CentOS Linux release 7.1.1503 (Core) > > CPU: Intel(R) Xeon(R) CPU E3-1245 v3 @ 3.40GHz > > Processor number: 8 > > Memory: 12GB > > fio command: > > 1. (for numjobs=64): > > fio --filename=/dev/md2 --sync=0 --direct=0 --rw=randread --bs=4K > --runtime=180 --size=50G --name=test-read --ioengine=libaio > --numjobs=64 --iodepth=1 --group_reporting > > 2. (for numjobs=1): > > fio --filename=/dev/md2 --sync=0 --direct=0 --rw=randread --bs=4K > --runtime=180 --size=50G --name=test-read --ioengine=libaio > --numjobs=1 --iodepth=1 --group_reporting > > > > Here are test results: > > > Part I. SSD (4 x 240GB Intel SSD create Raid6(syncing)) > > > a. 4K Random Read, numjobs=64 > > Average Throughput Average IOPS > > Kernel 3.19.8 715937KB/s 178984 > > Kernel 4.2.8 489874KB/s 122462 > > Kernel 4.2.8 Patch Rollback 717377KB/s 179344 > > > > b. 4K Random Read, numjobs=1 > > Average Throughput Average IOPS > > Kernel 3.19.8 32203KB/s 8051 > > Kernel 4.2.8 2535.7KB/s 633 > > Kernel 4.2.8 Patch Rollback 31861KB/s 7965 > > > > > Part II. HDD (4 x 1TB TOSHIBA HDD create Raid6(syncing)) > > > a. 4K Random Read, numjobs=64 > > Average Throughput Average IOPS > > Kernel 3.19.8 2976.6KB/s 744 > > Kernel 4.2.8 2915.8KB/s 728 > > Kernel 4.2.8 Patch Rollback 2973.3KB/s 743 > > > > b. 4K Random Read, numjobs=1 > > Average Throughput Average IOPS > > Kernel 3.19.8 481844 B/s 117 > > Kernel 4.2.8 24718 B/s 5 > > Kernel 4.2.8 Patch Rollback 460090 B/s 112 > > > > Thanks, > > -- > > Chien Lee
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