Re: Read errors on raid5 ignored, array still clean .. then disaster !!

[Giovanni Tessore <giotex@texsoft.it>]

> RAID-5 unfortunately is inherently insecure, here is why:
> If one drive gets kicked, MD starts recovering to a spare.
> At that point any single read error during the regeneration (that's a 
> scrub) will fail the array.
> This is a problem that cannot be overcome in theory.
Yes, I was just getting the same conclusion :-(
Suppose you have 2Tb mainstream disks, with a read error ratio of 1 
sector each 1E+14 bits = 1.25E+13 bytes.
It means that you likely get an error every 6.25 times you read the 
whole disk!
So in case of failure of a disk, you have 1 possibility over 6 to fail 
the array during recostruction.
Simply unacceptable!

I looked at specs of some enterprise disks, and the read error ratio for 
them is 1 sector each 1E+15 or each 1E+16. Better but still risky.
Ok.. I'll definitely move to raid-6.

Also raid-1 with less than 3 disks becomes useless the same way :-(
Idem for raid-10 ...wow

Well, these two threads on read errors came out as kinda instructive ... 
doh!!

Regards

-- 
Cordiali saluti.
Yours faithfully.

Giovanni Tessore