This off course is very logical... Raid5 writes to all 3 disks at about the same time plus it has to write the crc/verification data which also causes some overhead. so the average speed = 55+71+75 / 3 = 67... So your speed measurement is correct... Check out the difference from your raid0 config... Raid0 writes to all disks simultaneously (so if you write 100mb it is 3 x 33,3mb on each disk) If you add more disks your array does not necessarily have to become faster because of the overhead needed to be calculated... http://en.wikipedia.org/wiki/Standard_RAID_levels .. On Tue, 18 Sep 2007 09:12:31 -0700 (PDT) "Dean S. Messing" <deanm@xxxxxxxxxxxxx> 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, ~71, > and ~75 MB/sec. > > Shouldn't this array be running (at the slowest) > at about 55+71 = 126 MB/sec? I defined a RAID0 > on the ~55 and ~71 partitions and got > about 128 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: > > # 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 > > -- > fedora-list mailing list > fedora-list@xxxxxxxxxx > To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list
