Diagnosing slow disk write performance

Daniel Fussell dfussell at byu.edu
Fri Feb 28 11:48:21 MST 2014


On 02/27/2014 06:11 PM, Mike Lovell wrote:
> Hey everyone,
> I have a co-worker that just got a new system and is having some
> trouble with the write performance of both of the drives in his
> system. One is a large hard drive and one is a SSD. On both drives he
> can read sequential data at over 100MB/sec. The write performance is
> horrible though. Both drives are only able to do sequential write at
> about 1.3MB/sec. I have checked the data alignment and both drives
> have partitions and filesystems aligned at 4K boundaries. He is
> running Ubuntu 12.04 with a 3.11 kernel. The hardware is a ASRock z77
> motherboard that has Intel and Asmedia SATA controllers. Changing
> which controller the drive is connected to doesn't help nor does
> changing whether it is the 3Gbps or 6Gbps ports. The controllers are
> in AHCI mode. I don't remember what filesystem he is using but I'm
> guessing it is ext4 since that is the default in Ubuntu. There aren't
> any errors from the kernel and the SMART data says both drives are
> healthy. Do any of you have any suggestions as to things that might be
> causing this? Or things that could be tried to fix this?
>
> mike
>
What are you using to benchmark the read/write performance? 
Have you checked iostat -xm, specifically for the % utilization, service
time, average wait and io requests merged per second while running the
benchmark? 
Do the drives have write caching turned on? 
Are you writing in at least 4k blocks to the hard drive, or in page-size
blocks for the SSD?
Is the ext4 filesystem using fully journaled writes, or just metadata
journaling?
What does the kernel report for the drive's physical_block_size and
minimum_io_size?
What does hdparm -tT /dev/<device> say?



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