Re: Re: [PATCH 0/2] Improve odirect-write performance for block-device.

[majianpeng <majianpeng@gmail.com>]

On 2012-07-16 11:29 Shaohua Li <shli@kernel.org> Wrote:
>2012/7/15 majianpeng <majianpeng@gmail.com>:
>> 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)).
>
>>   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.