raid1 with sata is slow (speed of a single disc)

[peterh]

Binary package hint: mdadm

I use feisty

Reading from my raid1 is as fast as a single sata disc:

hdparm -t /dev/sda1

/dev/sda1:
 Timing buffered disk reads: 172 MB in 3.00 seconds = 57.27 MB/sec

hdparm -t /dev/sdb1

/dev/sdb1:
 Timing buffered disk reads: 172 MB in 3.01 seconds = 57.20 MB/sec

hdparm -t /dev/md0

/dev/md0:
 Timing buffered disk reads: 172 MB in 3.00 seconds = 57.29 MB/sec

i tried

* different motherboard with a different sata onboard controller but also the same results

* different ubuntu kernels: 2.6.17, 2.60.20 and from gutsy 2.6.22rc

* different images:
  linux-image-2.6.20-16-generic 2.6.20-16.29
  linux-image-2.6.20-16-lowlatency 2.6.20-16.29
  linux-image-2.6.20-16-server 2.6.20-16.29

can anybody explain me why?

[Loïc Minier (lool)]

I also witness this since forever; I think this is supposed to be normal for RAID 1 and people are supposed to use RAID 0+1 to get similar speed benefits as for RAID 0.

I'm also curious of the underlying technical reason.

[Loïc Minier (lool)]

So, section 1.4 of http://tldp.org/HOWTO/Software-RAID-HOWTO-1.html has the beginning of an answer: instead of optimizing performance for a single reader (hdparm -t), performance is optimized for multiple concurrent readers (two processes accessing different data).

I tried running two "dd" on two parts of the same RAID 1 array (with iflag=direct and and skip=<block> for the second dd) and got about 69.5 MB/s peek cumulated performance according to "killall -USR1 dd", while the drives are 42.5 and 49 MB/s capable (according to hdparm -t).