[ale] The perpetual question: best current HDD?

[Phil Turmel]

On 01/09/2013 11:30 PM, Brian Mathis wrote:
> There is a potential issue with raid check, at least on the older
> ones.  They are basically useless for any type of mirrored raid,
> except for finding out if it's bad.  The problem is that when you have
> 2 disks that are mirrored, and the data on them doesn't match, which
> one wins?  There's no way to know.  RAID5 can use its parity info to
> figure it out an rebuild, but you can't do this with a simple mirror.

This is not correct.  Raid5 *cannot* determine which drive is incorrect,
or even if multiple drives are incorrect.  Consider a write to a stripe,
with parity, that is interrupted by a power loss:  some drives may have
succeeded before their power died, and some not.  You cannot tell which
is which.

Raid6 suffers from the same problem, except it can statistically predict
which drive is wrong in a single-drive flaw.  But multi-drive flaws can
appear to be single-drive flaws, so it is still not certain.  The
author/maintainer of linux raid has a long blog post on the topic:

http://neil.brown.name/blog/20100211050355

Bottom line:  Mirrors are rebuilt from the first member.  Parity arrays
are rebuilt assuming the data blocks are correct and the parity blocks
wrong.

> By the time you find out you have this kind of bad block, it's too
> late.  So something like badblocks doing a full surface scan and
> smartd to warn you about relocated sectors can help you avoid getting
> to that point.

Scrubbing is all about finding transient UREs and rewriting them,
triggering relocation if necessary.  Mismatch detection is a side
benefit that can flag an array for a closer look.  A read-only badblocks
scan is insufficient, as it cannot rewrite when it finds errors, and
otherwise redundant if you are scrubbing, as scrubbing reads everything
except the tiny unused metadata area.

Along with scrubbing, regular background long SMART selftests are highly
recommended.  Use both mdmon and smartd for notifications.

Phil