Re: sata promise and software raid5...

[Willy Tarreau]

Hi,

I think I had the same problem on a PIIX SATA with RAID1. Basically, I noticed
that each disk could be accessed one at a time, and that it was totally
impossible to access the second one while the first one was reading. A simple
"cat /dev/sda >/dev/null" prevented anyone from logging into the box or doing
anything which required disk I/O.

So if you have the same problem, I can fairly understand your low numbers.
Could you try something like :

# cat /dev/sda >/dev/null &
# hdparm -t /dev/sdb

If hdparm doesn't start before the 'cat' ends, you do have the same problem.
Well, I don't know where it comes from, and unfortunately I don't have the
machine anymore to try fixes.

Regards,
Willy

On Wed, May 26, 2004 at 03:46:57AM +0400, Andrey Nekrasov wrote:
> Hello,
> 
> controller - Promise SATA 150 TX4, linux 2.6.7-rc1-bk1, HDD Maxtor
> 250Gb (x4)
> 
> Why so slowly reads (and writes) with software raid5?
> 
> gnome:~ # hdparm -t /dev/sda2
> 
> /dev/sda2:
>  Timing buffered disk reads:  148 MB in  3.01 seconds =  49.16 MB/sec
> gnome:~ # hdparm -t /dev/sdb2
> 
> /dev/sdb2:
>  Timing buffered disk reads:  148 MB in  3.02 seconds =  49.01 MB/sec
> gnome:~ # hdparm -t /dev/sdc2
> 
> /dev/sdc2:
>  Timing buffered disk reads:  148 MB in  3.03 seconds =  48.90 MB/sec
> gnome:~ # hdparm -t /dev/sdd2
> 
> /dev/sdd2:
>  Timing buffered disk reads:  144 MB in  3.00 seconds =  47.96 MB/sec
> gnome:~ #
> gnome:~ #
> gnome:~ # hdparm -t /dev/md0
> 
> /dev/md0:
>  Timing buffered disk reads:   64 MB in  3.12 seconds =  20.52 MB/sec
> gnome:~ #
> 
> 
> gnome:~ # cat /etc/raidtab
> raiddev /dev/md0
>         raid-level              5
>         nr-raid-disks           4
>         nr-spare-disks          0
>         persistent-superblock   1
>         parity-algorithm        left-symmetric
>         chunk-size              1024
> 
>         device                  /dev/sda2
>         raid-disk               0
> 
>         device                  /dev/sdb2
>         raid-disk               1
> 
>         device                  /dev/sdc2
>         raid-disk               2
> 
>         device                  /dev/sdd2
>         raid-disk               3
> 
>         
> bye
> --
> 
> 
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