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Re: Help: very slow software RAID 5.



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 sharplabs com> 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
> 
> -- 
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