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

From: Ric Wheeler
Date: Wed Aug 26 2009 - 06:39:38 EST

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.

- Ted

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.

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