Re: RAID timeout parameter accessibility request
From: Bill Davidsen
Date: Wed Jan 02 2008 - 12:55:42 EST
Jose de la Mancha wrote:
Hi everyone. I'm sorry but I'm not currently subscribed to this list (I've
been sent here by the listmaster), so please CC me all your
answers/comments. Thanks in advance.
SHORT QUESTION :
In a Debian-controlled RAID array, is there a parameter that handles the
timeout before a non-responding drive is dropped from the array ? Can this
timeout become user-adjustable in a future build ?
EXPLANATIONS :
As you might know, if you install and use a "desktop edition" hard drive in
a RAID array, the drive may not work correctly. This is caused by the normal
error recovery procedure that a desktop edition hard drive uses : when an
error is found on a desktop edition hard drive, the drive will enter into a
deep recovery cycle to attempt to repair the error, recover the data from
the problematic area, and then reallocate a dedicated area to replace the
problematic area. This process can take up to 120 seconds depending on the
severity of the issue.
The problem is that most RAID controllers allow a very short amount of time
(7-15 seconds) for a hard drive to recover from an error. If a hard drive
takes too long to complete this process, the drive will be dropped from the
RAID array !
Of course there are "RAID edition" hard drives with a feature called TLER
(Time Limited Error Recovery) which stops the hard drive from entering into
a deep recovery cycle. The hard drive will only spend 7 seconds to attempt
to recover. This means that the hard drive will not be dropped from a RAID
array. But these "special" hard drives are way too expensive IMHO just for a
small firmware-based feature.
I'm not sure "way too expensive" is appropriate. I'm using Seagate 320GB
"NS" drives instead of "AS" models, and at the time I bought them they
were $100 vs. $95 and are now $95 vs. $85. Other sizes and vendors have
similar price points. A 10-15% premium seems reasonable for the
firmware, faster access, and a general assurance that the drive is
intended for 7/24 use.
On a small home array the cost is minimal, and if you are running
desktop drives in huge arrays 7/24 you probably are doing other cost
cutting tradeoffs to reliability, it's a choice.
There would be an easy way to allow users to use "ordinary" hard drives in a
Debian software-controlled RAID array. So here's my request : I suppose
there is a parameter that handles the default timeout before a drive is
dropped from the RAID array. I don't know if this parameter is hardcoded,
but it would be nice if it was user-adjustable. This way, we could simply
set up this parameter to 120 seconds or more (instead of 7-15) and we
wouldn't have any more problems with using desktop "edition hard" drives in
a RAID array.
What do you think ? Can it be done in a future build ?
Just in general I agree, I'd like to see the error reported back up and
let the user make the decision. One of the benefits of Linux is that it
lets you make your own decisions, even bad ones. Windows assumes it owns
the computer, you're an idiot, and it will let you use your computer if
you do it their way.
I really hope that you'll be able to help, because I guess a lot of people
can be concerned by this issue.
Not so much, with rewrites of sectors where possible this problem is
less common than it was. But I agree on the drive kicking option, more
control is good. However, the timeout should be in the driver, not in
the raid code, that's where it belongs. The kernel copes with errors
better than having a drive go practice self-gratification for minutes at
a time.
--
Bill Davidsen <davidsen@xxxxxxx>
"We have more to fear from the bungling of the incompetent than from
the machinations of the wicked." - from Slashdot
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