> I am currently experimenting with the Software RAID (patch for 2.2.11
> applied to 2.2.12 with raidtools snapshot from about three weeks ago).
Same as I am running here.
> The system I am doing these tests on is an older K5-100
- slow CPU
For RAID5 you need quite some CPU power (esp. if writing to the array).
For RAID0 or RAID1, this is no problem.
> For RAID, I have three Quantum Atlas I
Don't know these HDDs from practical experience ...
> on a single Fast SCSI Host Adapter with NCR Chipset.
- slow hostadapter, only one channel for 3 HDDs - theoretical limit is 10MB/s.
> This test is meant to check write rate of the RAID but will measure
> source disk performance instead because the source disk is likely to
> be the bottle neck here.
Yes ;-)
> "Copy 2" is the time taken for "cp -R 1 2" , with both source and
> target directory on the RAID array. I hope to simulate a realistic
> usage profile with many small files, read and write operations.
More or less a seek time test, too.
> Second source of astonishment is that the RAID array is quite slower
> than a single disk.
It can't get much faster because of your SCSI controller (<= 10MB/s) and CPU
(maybe limiting, too !?).
If you use RAID5, you also must consider, that MORE data is stored on disks
than size of files you copied to the disks (well: REDUNDANT data). So if you
have 3 disks with RAID5 and store a 10MB file on them, 15MB data will have to
be written to disk. If your SCSI bus could make full 10MB/s you would only see
6.6 MB/s of them (in theory).
> the RAID array can only be used as a safety measure to be able to
> recover from a drive failure for system and home directories.
In your case, yes. Maybe get 2 more SCSI controllers of same type (or U(2)W ctlr
+ disks), put 1 disk on each channel and try RAID0 for better performance.
Just some performance values I got with my RAID arrays:
3 disks IBM DNES 9GB UW on single UW channel, Dual Celeron 333MHz, SW RAID5:
this gives about 17MB/s bonnie (block read and write).
4 disks IBM DNES 9GB UW on 2 UW channels, Dual Celeron 500 MHz, 100MHz FSB ;-),
SW RAID5: this gives about 25MB/s bonnie (block read and write)
3 disks IBM DDRS (or DNES !?) 9GB UW on 2 channels, Dual Celeron ???MHz,
SW RAID0: this gave me about 40MB/s bonnie (block) !
Thomas
--
Thomas Waldmann (com_ma, Computer nach Masz)
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