Old question? Performance in raid 1 configuration

Peter Boy pboy at barkhof.uni-bremen.de
Wed Aug 10 15:11:29 UTC 2005

Hi all,

sorry if I ask an old question, but I couldn't find a definite answer on
the web.

I just bought some new computer (shuttle SN85 / AMD 64 / nforce3 / 2
sata HD), which should be configured as raid 1 (mirroring).

Question is should I simply use Linux Software Raid or should I try to
use dmraid and the nforce chip (Fedora Core 4 and RHEL WS 4, both seem
quite difficult to configure)?

I know, many people recommend Linux md/sw raid. Arjan argued for this
several times and based his argument mainly on theoretical
considerations. There are some findings 
  (e.g. http://www.jab.org/raid-bench/)
which show that md raid 1 does not perform worse than a single disk.
Others found that Linux md raid performs better as even a real hardware
raid ( http://www.chemistry.wustl.edu/~gelb/castle_raid.html ,
http://spamaps.org/raidtests.php ). But the several fake raid controller
seem to differ quite a lot.

OK. I tried to copy a 700 mb iso file and found:

disk -> disk:  19 sec.
disk -> raid:  39 sec.

Writing to the raided partition took roughly twice the time. Seems the
kernel itself has to write the file two times one after another. 


- Can/Could/Should the usage of dmraid/device mapper give a  
  better write performance as the md raid? (I hope the chip can
  write in parallel, but absolutely don't know anything about
  it's technical details and could find any information)

- If I use md/SW raid should I nevertheless activate the raid
  functionality in BIOS?

- If I later want to test dmraid, can I use the partitions and
  filesystems, configured for md/sw raid (with the auto raid block),
  or have I to clear and reconfigure the disks anyway?

Any information highly appreciated.

Thanks in advance

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