Messages in this thread | | | Date | Sat, 27 Feb 2010 19:09:46 -0500 | From | Bill Davidsen <> | Subject | Re: mdadm software raid + ext4, capped at ~350MiB/s limitation/bug? |
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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.
-- Bill Davidsen <davidsen@tmr.com> "We can't solve today's problems by using the same thinking we used in creating them." - Einstein
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