Recovering from an URE on a RAID5 rebuild/resize

[Roman Mamedov <rm@romanrm.ru>]

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Hello,

Recently there has been some talk on this list, about probability of seeing an
URE during a RAID5 rebuild on modern large (e.g. 2TB) drives.

I would like to ask for some advice of what would be the best way to proceed
when such an URE is encountered. This is mostly theoretical, no real situation
at hand at the moment.

As I understand, a RAID5 that is being resized or rebuilt, has no redundancy;
it is essentially as reliable as a RAID0 of total members-1, or even less.

So on an unreadable sector that mdadm needs to read (because it has no
redundancy to recover it from), mdadm will:

  - mark the corresponding array member as "failed";
  - mark the one that was being rebuilt/resized onto as "spare";
  - and the whole array as down and "not enough members to start the array".

Let's assume only a couple of sectors on that member were unreadable, and then
their readability was restored (either by drive replacement or by overwriting
them to making the drive remap), and I would be okay with losing data that was
in those sectors.

What would be the best way to proceed from there?


-- 
With respect,
Roman

~~~~~~~~~~~~~~~~~~~~~~~~~~~
"Stallman had a printer,
with code he could not see.
So he began to tinker,
and set the software free."

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