Re: [patch] ext2/3: document conditions when reliable operation ispossible

From: Ric Wheeler
Date: Wed Aug 26 2009 - 08:02:01 EST

On 08/26/2009 07:12 AM, Pavel Machek wrote:
On Wed 2009-08-26 06:39:14, Ric Wheeler wrote:
On 08/25/2009 10:58 PM, Theodore Tso wrote:
On Tue, Aug 25, 2009 at 09:15:00PM -0400, Ric Wheeler wrote:

I agree with the whole write up outside of the above - degraded RAID
does meet this requirement unless you have a second (or third, counting
the split write) failure during the rebuild.

The argument is that if the degraded RAID array is running in this
state for a long time, and the power fails while the software RAID is
in the middle of writing out a stripe, such that the stripe isn't
completely written out, we could lose all of the data in that stripe.

In other words, a power failure in the middle of writing out a stripe
in a degraded RAID array counts as a second failure.
To me, this isn't a particularly interesting or newsworthy point,
since a competent system administrator who cares about his data and/or
his hardware will (a) have a UPS, and (b) be running with a hot spare
and/or will imediately replace a failed drive in a RAID array.
I agree that this is not an interesting (or likely) scenario, certainly
when compared to the much more frequent failures that RAID will protect
against which is why I object to the document as Pavel suggested. It
will steer people away from using RAID and directly increase their
chances of losing their data if they use just a single disk.
So instead of fixing or at least documenting known software deficiency
in Linux MD stack, you'll try to surpress that information so that
people use more of raid5 setups?

Perhaps the better documentation will push them to RAID1, or maybe
make them buy an UPS?

I am against documenting unlikely scenarios out of context that will lead people to do the wrong thing.


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