Re: mdadm software raid + ext4, capped at ~350MiB/s limitation/bug?

[Justin Piszcz <jpiszcz@lucidpixels.com>]

On Sat, 27 Feb 2010, Bill Davidsen wrote:

> Justin Piszcz wrote:
>> 
>> 
>> On Sun, 28 Feb 2010, Neil Brown wrote:
>> 
>>> On Sat, 27 Feb 2010 08:47:48 -0500 (EST)
>>> Justin Piszcz <jpiszcz@lucidpixels.com> wrote:
>>> 
>>>> Hello,
>>>> 
>>>> I have two separate systems and with ext4 I cannot get speeds greater 
>>>> than
>>>> ~350MiB/s when using ext4 as the filesystem on top of a raid5 or raid0.
>>>> It appears to be a bug with ext4 (or its just that ext4 is slower for 
>>>> this
>>>> test)?
>>>> 
>>>> Each system runs 2.6.33 x86_64.
>>> 
>>> Could be related to the recent implementation of IO barriers in md.
>>> Can you try mounting your filesystem with
>>>   -o barrier=0
>>> 
>>> and see how that changes the result.
>>> 
>>> NeilBrown
>> 
>> Hi Neil,
>> 
>> Thanks for the suggestion, it has been used here:
>> http://lkml.org/lkml/2010/2/27/66
>> 
>> Looks like an EXT4 issue as XFS does ~600MiB/s..?
>> 
>> Its strange though, on a single hard disk, I get approximately the same 
>> speed for XFS and EXT4, but when it comes to scaling across multiple disks,
>> in RAID-0 or RAID-5 (tested), there is a performance problem as it hits a
>> performance problem at ~350MiB/s.  I tried multiple chunk sizes but nothing
>> seemed to made a difference (whether 64KiB or 1024KiB), XFS performs at
>> 500-600MiB/s no matter what and EXT4 does not exceed ~350MiB/s.
>> 
>> Is there anyone on any of the lists that gets > 350MiB/s on a mdadm/sw raid
>> with EXT4?
>> 
>> A single raw disk, no partitions:
>> p63:~# dd if=/dev/zero of=/dev/sdm bs=1M count=10240
>> 10240+0 records in
>> 10240+0 records out
>> 10737418240 bytes (11 GB) copied, 92.4249 s, 116 MB/s
>
> I hate to say it, but I don't think this measures anything useful. When I was 
> doing similar things I got great variabilty in my results until I learned 
> about the fdatasync option so you measure the actual speed to the destination 
> and not the disk cache. After that my results were far slower and 
> reproducible.

fdatasync:
http://lkml.indiana.edu/hypermail/linux/kernel/1002.3/01507.html