From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <650f1c31a83a452580882cbad2dfbba7@quanstro.net>
From: erik quanstrom <quanstro@quanstro.net>
Date: Mon, 21 Sep 2009 14:02:40 -0400
To: 9fans@9fans.net
In-Reply-To: <20090921174310.D5B4C5B55@mail.bitblocks.com>
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Subject: Re: [9fans] Petabytes on a budget: JBODs + Linux + JFS
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> > i would think this is acceptable.  at these low levels, something
> > else is going to get you -- like drives failing unindependently.
> > say because of power problems.
>
> 8% rate for an array rebuild may or may not be acceptable
> depending on your application.

i think the lesson here is don't by cheep drives; if you
have enterprise drives at 1e-15 error rate, the fail rate
will be 0.8%.  of course if you don't have a raid, the fail
rate is 100%.

if that's not acceptable, then use raid 6.

> > so there are 4 ways to fail.  3 double fail have a probability of
> > 3*(2^9 bits * 1e-14 1/ bit)^2
>
> Why 2^9 bits? A sector is 2^9 bytes or 2^12 bits.

cut-and-paste error.  sorry that was 2^19 bits, e.g. 64k*8 bits/byte.
the calculation is still correct, since it was done on that basis.

> If per sector recovery is done, you have
> 	3E-22*(64K/512) = 3.84E-20

i'd be interested to know if anyone does this.  it's not
as easy as it would first appear.  do you know of any
hardware or software that does sector-level recovery?

i don't have enough data to know how likely it is to
have exactly 1 bad sector.  any references?

- erik