Raid 5 (was: Mounting of Linux volumes)

[Nicholas Leippe]

On Thursday 01 December 2005 07:03 pm, Michael Torrie wrote:
> In any event, software RAID-5 does have significant issues to plan for
> and I would not recommend using it in a situation where hardware RAID-5
> is clearly the better way to go.  Even for home use, for $200 you can
> pick up a pretty decent SATA RAID controller.

With the most recent 2.6 kernels, and SCSI or SATA, most of the issues around 
sofware RAID-5 have been handled.  You can now hotswap drives even w/o a hw 
controller (you need hot swap drives though, of course).

The only real issues that I consider is lack of battery backed ram, and lack 
of control of write caching on the drives (maybe there's a proc/ or sys/ 
interface for this?).  (There are performance trade-offs of course, but if 
the system's primary use is storage, then I consider them mostly moot.)

As for battery backed ram, that only helps with a system power failure (all 
power supplies die, or your UPS fails w/o signaling for a clean shutdown).
(you _do_ have a UPS and redundant PSWs on your servers, no?)
AFAIK, I still don't trust the BBU ram on a hw controller to help when someone 
hits the reset button--doesn't reset tell the card to reset? (You'd hope the 
card maker thought of that, but seeing so many obvious things some have 
missed makes you wonder).  Perhaps reset isn't an issue with the controller 
card, but it is most surely an issue with drive write caches.  I have several 
hw raid controllers that don't allow it to be configured, and some don't even 
expose the status.

As for rsync'ing an entire drive, are you doing so with the source drive 
mounted rw?  Cause if so, you may not get a 'snapshot' of the drive.  
Depending on the apps using the drive, this could cause serious problems.


-- 
Respectfully,

Nicholas Leippe
Sales Team Automation, LLC
1335 West 1650 North, Suite C
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