Re: raid0 slower than devices it is assembled of?

[jw schultz <jw@pegasys.ws>]

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