Re: Filesystem writes on RAID5 too slow

[Eric Sandeen <sandeen@redhat.com>]

On 11/18/13, 10:02 AM, Martin Boutin wrote:
> Dear list,
> 
> I am writing about an apparent issue (or maybe it is normal, that's my
> question) regarding filesystem write speed in in a linux raid device.
> More specifically, I have linux-3.10.10 running in an Intel Haswell
> embedded system with 3 HDDs in a RAID-5 configuration.
> The hard disks have 4k physical sectors which are reported as 512
> logical size. I made sure the partitions underlying the raid device
> start at sector 2048.

(fixed cc: to xfs list)

> The RAID device has version 1.2 metadata and 4k (bytes) of data
> offset, therefore the data should also be 4k aligned. The raid chunk
> size is 512K.
> 
> I have the md0 raid device formatted as ext3 with a 4k block size, and
> stride and stripes correctly chosen to match the raid chunk size, that
> is, stride=128,stripe-width=256.
> 
> While I was working in a small university project, I just noticed that
> the write speeds when using a filesystem over raid are *much* slower
> than when writing directly to the raid device (or even compared to
> filesystem read speeds).
> 
> The command line for measuring filesystem read and write speeds was:
> 
> $ dd if=/tmp/diskmnt/filerd.zero of=/dev/null bs=1M count=1000 iflag=direct
> $ dd if=/dev/zero of=/tmp/diskmnt/filewr.zero bs=1M count=1000 oflag=direct
> 
> The command line for measuring raw read and write speeds was:
> 
> $ dd if=/dev/md0 of=/dev/null bs=1M count=1000 iflag=direct
> $ dd if=/dev/zero of=/dev/md0 bs=1M count=1000 oflag=direct
> 
> Here are some speed measures using dd (an average of 20 runs).:
> 
> device       raw/fs  mode   speed (MB/s)    slowdown (%)
> /dev/md0    raw    read    207
> /dev/md0    raw    write    209
> /dev/md1    raw    read    214
> /dev/md1    raw    write    212
> 
> /dev/md0    xfs    read    188    9
> /dev/md0    xfs    write    35    83
> 
> /dev/md1    ext3    read    199    7
> /dev/md1    ext3    write    36    83
> 
> /dev/md0    ufs    read    212    0
> /dev/md0    ufs    write    53    75
> 
> /dev/md0    ext2    read    202    2
> /dev/md0    ext2    write    34    84
> 
> Is it possible that the filesystem has such enormous impact in the
> write speed? We are talking about a slowdown of 80%!!! Even a
> filesystem as simple as ufs has a slowdown of 75%! What am I missing?

One thing you're missing is enough info to debug this.

/proc/mdstat, kernel version, xfs_info output, mkfs commandlines used,
partition table details, etc.

If something is misaligned and you are doing RMW for these IOs it could
hurt a lot.

-Eric

> Thank you,
>