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.
Question:
- 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
Peter
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