Help, my disk array has one dead member

[Karl Auer]

On Sat, 2017-03-25 at 13:59 +0100, Xen wrote:
> Karl Auer schreef op 24-03-2017 4:02:
> > If you go for RAID5, be aware that it is fairly slow to write. Very
> > safe though, and good error recovery. RAID6 is even safer, and
> > should be considered once you start to get much over 5 or 6
> > terabytes, because the error rates even on modern drives start to
> > make bit errors pretty much inevitable.
> Normally sectors on drives (both 512 and 4k) are prefixed with error 
> correction codes, aren't they?

Yes - and if the sector is gone, so is the error correction. That's why
RAID stores error correction on different blocks, on different drives
and (in the case of RAID6) multiple times. And the error correction in
RAID covers multiple blocks, not just one, so you can recover a whole
drive if you lose a RAID5 drive. RAID1 takes a different approach and
just duplicates entire disks.

> That's the reason 4k is more efficient than 512.

I don't think so, but I'm not sure. I think the main reason larger
blocks are more efficient is because larger single writes can be
managed by the drive.

> How common is bit corruption during file transfer on say a regular
> hard disk?

It's all about the statistical likelihood of an unrecoverable bit
error, which drive manufacturers test and report as the bit error rate
(BER). If you have a drive with a BER of 10^14 (the standard in
consumer drives) it means that the manufacturer reports a likelihood of
1 in 10^14 that a bit will be read with the wrong value.

Read this for an influential article written by Rob Harris in 2007:

http://www.zdnet.com/article/why-raid-5-stops-working-in-2009/

... but don't stop there. Plenty of people disagree with him, for
example:

https://www.high-rely.com/blog/using-raid5-means-the-sky-is-falling/

Basically the discussion starts with manufacturers reporting
unrecoverable bit error rates of 1x10^14, or roughly 1 in 12.5
terabytes read.

The main thing to take away from the discussion IMHO is that RAID is no
substitute for backup.

Regards, K.

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Karl Auer (kauer at biplane.com.au)
http://www.biplane.com.au/kauer
http://twitter.com/kauer389

GPG fingerprint: A52E F6B9 708B 51C4 85E6 1634 0571 ADF9 3C1C 6A3A
Old fingerprint: E00D 64ED 9C6A 8605 21E0 0ED0 EE64 2BEE CBCB C38B