2012/7/15 majianpeng <majianpeng@xxxxxxxxx>: > On 2012-07-16 11:29 Shaohua Li <shli@xxxxxxxxxx> Wrote: >>2012/7/15 majianpeng <majianpeng@xxxxxxxxx>: >>> Create a raid5 using four disk and the chunksize is 512K. >>> Test command is: dd if=/dev/zero of=/dev/md0 bs=1536K count=90000 oflag=direct >>> >>> In RHEL6(kernel 2.6.32):speed about 240MB/s >>> In 3.5.0-rc5:speed about 77MB/S >>> Add two patch in 3.5.0-rc5, speed about 200MB/S. >>> >>> So the performance of odirect-wrirte for block-deivce was obvious reduced. >>> PATCH 1/2: Add blk_plug function for odirect-write block-device >>> PATCH 2/2: Remove REQ_SYNC for odirect-write in raid456. >>> >>> PATCH 2/2 maybe not correct because it alse for odirect-write for regular file. >>> Jianpeng Ma (2): >>> fs/block-dev.c:fix performance regression in O_DIRECT writes to >>> md block devices. >> >>In raid5, all requests are submitted by raid5d thread, which already has >>plug. Why doesn't it work? > No. the purpose of two patch is to reduce the read operation when write which was not full-write. > I tested in RHEL6.The read operation is zero.But in 3.5.0-rc5, the read operaiton may equal to write-operation. > And i used the bs was 1536k(3*512k(chunk-size)). yes, I know. But I want to understand why we need the plug in your test. The IO is dispatched from raid5d, it already has plug. Fengguang used to post a patch to move the plug from generic_file_aio_write to do_blockdev_direct_IO, which sounds better. >>> raid5: For write performance, remove REQ_SYNC when write was odirect. >> >>REQ_SYNC only impacts CFQ, this sounds not reasonable. So the disks >>are using CFQ ioscheduler. Can you check if you can see the same issue >>with deadline? > I tested and the result is the same like cfq. > But in RHEL6, the ioscheduler is also cfq. >> >>Let me guess, without REQ_SYNC, read will get higher priority against write >>in CFQ, so in this case, write gets delayed, and maybe get better write >>request merge. And now with REQ_SYNC, read and write has the same >>priority, there is less request merge. >> >>Thanks, >>Shaohua > For harddisk,the read for not full-write will remarkly reduce the performance. > So the first it to make write full-write as posible. yes, this is the symptom, but I'd like to understand why REQ_SYNC makes the difference. Thanks, Shaohua -- 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
