On Tue, 18 Sep 2007, Dean S. Messing wrote:
I'm not getting nearly the read speed I expected
from a newly defined software RAID 5 array
across three disk partitions (on the 3 drives,
of course!).
Would someone kindly point me straight?
After defining the RAID 5 I did `hdparm -t /dev/md0'
and got the abysmal read speed of ~65MB/sec.
The individual device speeds are ~55, ~70,
and ~75 MB/sec.
Shouldn't this array be running (at the slowest)
at about 55+70 = 125 MB/sec minus some overhead?
I defined a RAID0 on the ~55 and ~70 partitions
and got about 110 MB/sec.
Shouldn't adding a 3rd (faster!) drive into the
array make the RAID 5 speed at least this fast?
Here are the details of my setup:
Linux Fedora 7, kernel 2.6.22.
# fdisk -l /dev/sda
Disk /dev/sda: 160.0 GB, 160000000000 bytes
255 heads, 63 sectors/track, 19452 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sda1 1 127 1020096 82 Linux swap / Solaris
/dev/sda2 * 128 143 128520 83 Linux
/dev/sda3 144 19452 155099542+ fd Linux raid autodetect
# fdisk -l /dev/sdb
Disk /dev/sdb: 160.0 GB, 160000000000 bytes
255 heads, 63 sectors/track, 19452 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sdb1 * 1 127 1020096 82 Linux swap / Solaris
/dev/sdb2 128 143 128520 83 Linux
/dev/sdb3 144 19452 155099542+ fd Linux raid autodetect
# fdisk -l /dev/sdc
Disk /dev/sdc: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sdc1 * 1 127 1020096 82 Linux swap / Solaris
/dev/sdc2 128 19436 155099542+ fd Linux raid autodetect
/dev/sdc3 19437 60801 332264362+ 8e Linux LVM
The RAID 5 consists of sda3, sdb3, and sdc2.
These partitions have these individual read speeds:
# hdparm -t /dev/sda3 /dev/sdb3 /dev/sdc2
/dev/sda3:
Timing buffered disk reads: 168 MB in 3.03 seconds = 55.39 MB/sec
/dev/sdb3:
Timing buffered disk reads: 216 MB in 3.03 seconds = 71.35 MB/sec
/dev/sdc2:
Timing buffered disk reads: 228 MB in 3.02 seconds = 75.49 MB/sec
After defining RAID 5 with:
mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sda3 /dev/sdb3 /dev/sdc2
and waiting the 50 minutes for /proc/mdstat to show it was finished,
I did `hdparm -t /dev/md0' and got ~65MB/sec.
Dean
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Without tuning you will get very slow read speed.
Read the mailing list, there are about 5-10 tunable options, for me, I get
250 MiB/s no tuning (read/write), after tuning 464 MiB/s write and 622
MiB/s read.
Justin.
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