Re: Incredibly poor performance of mdraid-1 with 2 SSD Samsung 840 PRO

[Stan Hoeppner <stan@hardwarefreak.com>]

On 4/21/2013 7:23 AM, Tommy Apel wrote:
> Hello, FYI I'm getting ~68MB/s on two intel330 in RAID1 aswell on
> vanilla 3.8.8 and 3.9.0-rc3 when writing random data and ~236MB/s
> writing from /dev/zero
> 
> mdadm -C /dev/md0 -l 1 -n 2 --assume-clean --force --run /dev/sdb /dev/sdc


> openssl enc -aes-128-ctr -pass pass:"$(dd if=/dev/urandom bs=128
> count=1 2>/dev/null | base64)" -nosalt < /dev/zero | pv -pterb >
> /run/fill ~1.06GB/s

What's the purpose of all of this?  Surely not simply to create random
data, which is accomplished much more easily.  Are you sand bagging us
here with a known bug, or simply trying to show off your mad skillz?
Either way this is entirely unnecessary for troubleshooting an IO
performance issue.  dd doesn't (shouldn't) care if the bits are random
or not, though the Intel SSD controller might, as well as other layers
you may have in your IO stack.  Keep it simple so we can isolate one
layer at a time.

> dd if=/run/fill of=/dev/null bs=1M count=1024 iflag=fullblock ~5.7GB/s
> dd if=/run/fill of=/dev/md0 bs=1M count=1024 oflag=direct ~68MB/s
> dd if=/dev/zero of=/dev/md0 bs=1M count=1024 oflag=direct ~236MB/s

Noting the above, it's interesting that you omitted this test

  dd if=/run/fill of=/dev/sdb bs=1M count=1024 oflag=direct

preventing an apples to apples comparison between raw SSD device and
md/RAID1 performance with your uber random file as input.

-- 
Stan