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SubjectRe: raid0 slower than devices it is assembled of?
On Mon, Dec 15, 2003 at 02:34:54PM +0100, Witold Krecicki wrote:
> I've got / on linux-raid0 on 2.6.0-t11-cset-20031209_2107:
> <cite>
> /dev/md/1:
> Version : 00.90.01
> Creation Time : Thu Sep 11 22:04:54 2003
> Raid Level : raid0
> Array Size : 232315776 (221.55 GiB 237.89 GB)
> Raid Devices : 2
> Total Devices : 2
> Preferred Minor : 1
> Persistence : Superblock is persistent
>
> Update Time : Mon Dec 15 12:55:48 2003
> State : clean, no-errors
> Active Devices : 2
> Working Devices : 2
> Failed Devices : 0
> Spare Devices : 0
>
> Chunk Size : 64K
>
[snip]

> Disks are two ST3120026AS connected to sii3112a controller, driven by sata_sil
> 'patched' so no limit for block size is applied (it's not needed for it).
>
> Those are results of hdparm -tT on drives:
> <cite>
> /dev/md/1:
> Timing buffer-cache reads: 128 MB in 0.40 seconds =323.28 MB/sec
> Timing buffered disk reads: 64 MB in 1.75 seconds = 36.47 MB/sec
> /dev/sda:
> Timing buffer-cache reads: 128 MB in 0.41 seconds =309.23 MB/sec
> Timing buffered disk reads: 64 MB in 1.46 seconds = 43.87 MB/sec
> /dev/sdb:
> Timing buffer-cache reads: 128 MB in 0.41 seconds =315.32 MB/sec
> Timing buffered disk reads: 64 MB in 1.23 seconds = 52.04 MB/sec
> </cite>
> What seems strange to me is that second drive is faster than first one
> (devices are symmetrical, sd[a,b]2 is swapspace (not mounted at time of
> test), sd[a,b]1 is /boot (raid1)).

Possible reasons:

internal differences on controller

block remapping (even new disks have bad blocks)

different firmware
different physical geometry -- two production runs of
the same make+model drive may have different
geometry
cable quality or routing differences, or interface
variations that cause subtle timing differences


> What is even stranger is that raid0 which should be faster than single drive,
> is pretty much slower- what's the reason of that?

You could try increasing the read ahead but that may slow
things down in real world use.

AID-0 isn't RAID (no R), but then again for many arrays the
I is also out of place.

--
________________________________________________________________
J.W. Schultz Pegasystems Technologies
email address: jw@pegasys.ws
Remember Cernan and Schmitt
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