Re: 4 disks in raid 5: 33MB/s read performance?

[Dexter Filmore <Dexter.Filmore@gmx.de>]

> On Monday May 22, Dexter.Filmore@gmx.de wrote:
> > I just dd'ed a 700MB iso to /dev/null, dd returned 33MB/s.
> > Isn't that a little slow?
> > System is a sil3114 4 port sata 1 controller with 4 samsung spinpoint
> > 250GB, 8MB cache in raid 5 on a Athlon XP 2000+/512MB.
>
> Yes, read on raid5 isn't as fast as we might like at the moment.
>
> It looks like you are getting about 11MB/s of each disk which is
> probably quite a bit slower than they can manage (what is the
> single-drive read speed you get dding from /dev/sda or whatever).
>
> You could try playing with the readahead number (blockdev --setra/--getra).
> I'm beginning to think that the default setting is a little low.

Changed from 384 to 1024, no improvement.

>
> You could also try increasing the stripe-cache size by writing numbers
> to
>    /sys/block/mdX/md/stripe_cache_size

Actually, there's no directory /sys/block/md0/md/ here. Can I find that in 
proc somewhere? And what are sane numbers for this setting?

> I wonder if your SATA  controller is causing you grief.
> Could you try
>    dd if=/dev/SOMEDISK of=/dev/null bs=1024k count=1024
> and then do the same again on all devices in parallel
> e.g.
>    dd if=/dev/SOMEDISK of=/dev/null bs=1024k count=1024 &
>    dd if=/dev/SOMEOTHERDISK of=/dev/null bs=1024k count=1024 &
>    ...

 4112 pts/0    R      0:00 dd if /dev/sda of /dev/null bs 1024k count 1024
 4113 pts/0    R      0:00 dd if /dev/sdb of /dev/null bs 1024k count 1024
 4114 pts/0    R      0:00 dd if /dev/sdc of /dev/null bs 1024k count 1024
 4115 pts/0    R      0:00 dd if /dev/sdd of /dev/null bs 1024k count 1024
 4116 pts/0    R+     0:00 ps ax

1024+0 Datensätze ein
1024+0 Datensätze aus
1073741824 Bytes (1,1 GB) kopiert, 34,5576 Sekunden, 31,1 MB/s
1024+0 Datensätze ein
1024+0 Datensätze aus
1073741824 Bytes (1,1 GB) kopiert, 36,073 Sekunden, 29,8 MB/s
1024+0 Datensätze ein
1024+0 Datensätze aus
1073741824 Bytes (1,1 GB) kopiert, 40,5109 Sekunden, 26,5 MB/s
1024+0 Datensätze ein
1024+0 Datensätze aus
1073741824 Bytes (1,1 GB) kopiert, 40,5054 Sekunden, 26,5 MB/s

(Partly german, but I think you get it)

A single disks pumps out 65-70MB/s. Since they are on a PCI 32bit controller 
the combined speeds when reading from all four disks at once pretty much max 
the 133MB/s PCI limit. (I' surprised it comes so close. That controller works 
pretty well for 18 bucks.)

Dex


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