[kwlug-disc] Hardware raid vs linux
unsolicited at swiz.ca
Thu Aug 20 20:39:13 EDT 2009
The one very significant advantage is you can take the disks to any
system and keep on going. With hardware raid, you must take the
controller with you. In that sense, software RAID is more redundant
than hardware RAID, in a sense. At least once I've seen hardware RAID
fail, it was the controller, and we couldn't buy another. Back to
backups we had to go. (Insidious.)
Others will have to point you to reliability statistics, etc.
A couple of questions to ask yourself:
(1) How fast is fast enough? e.g. my home GB network is truly useful
(vs 10/100 MB) for machine to machine backups in the house. (I'm
talking hours, if not days, of difference in transfer times.) For the
internet, not so much. I don't expect to ever have >= 10 Mbps internet
at home, so internally a network faster than 10 Mbps doesn't buy me
anything - except for serving streams within the house, backups, etc.
Similarly, if software RAID is sufficiently fast to deliver to your
www requestors on your < 1 TBps internet connection, then does any
speed increase via hardware RAID really buy you anything? So, is
software RAID fast enough?
(2) Regardless of the RAID type, the point of RAID is hardware
redundancy and up time (hot swap). So, are your mounted drives
externally accessible / hot swappable? What I'm getting at is, if they
aren't, you're going to have to take the machine down to swap broken
drives anyways. So is RAID really going to buy you anything, anyways?
[vs. a couple really big drives that manually mirror to each other -
Everyone's installation is different, so there's no one right answer.
John (& others?) made comments on the list not long ago that hardware
RAID was taking them too long to rebuild, so they gave up on RAID
entirely. In favour of some form of mirroring.
In your poking about for information, you might keep an eye out for
ease of rebuilding issues. e.g. Frequently RAID can be run in a
degraded state (1 drive failed, but keeps going in the meantime), and
can rebuild the same way (keeps serving while rebuilding the array) -
so although you're slower for the longer duration, at least you're
serving something. (Some think the degradation too much, so rebuild
off-line, leading to John's comments.) Some RAIDs will continue a
rebuild after a boot (perhaps some won't?), so an e-mail server, for
example, we'll off-line rebuild as much as we can from close of
business one day through to opening of business the next, at which
point we'll reboot to online rebuild. Slower, but at least some
service. I'm not familiar with, or if, Linux software RAID works the
same way - you might want to keep an eye open.
Insurance Squared Inc. wrote, On 08/20/2009 7:47 PM:
> I'm looking at rebuilding raid on my webserver which provides for
> hardware raid. Reading online seems to indicate that I'd be better off
> using software raid under linux (linux, is there anything you can't
> do?). Seems a bit suspicious to me that software outperforms hardware.
> Any comments on the reality of the situation?
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