Re: [9fans] Petabytes on a budget: JBODs + Linux + JFS

[erik quanstrom <quanstro@quanstro.net>]

> I am going to try my hands at beating a dead horse:)
> So when you create a Venti volume, it basically writes '0's' to all the
> blocks of the underlying device right?  If I put a venti volume on a AoE
> device which is a linux raid5, using normal desktop sata drives, what
> are my chances of a successful completion of the venti formating (let's
> say 1TB raw size)?

drive mfgrs don't report write error rates.  i would consider any
drive with write errors to be dead as fried chicken.  a more
interesting question is what is the chance you can read the
written data back correctly.  in that case with desktop drives,
you have a
	8 bits/byte * 1e12 bytes / 1e14 bits/ure = 8%
i'm a little to lazy to calcuate what the probabilty is that
another sector in the row is also bad.  (this depends on
stripe size, the number of disks in the raid, etc.)  but it's
safe to say that it's pretty small.  for a 3 disk raid 5 with
64k stripes it would be something like
	8 bites/byte * 64k *3 / 1e14 = 1e-8
i'm making the completely unwarrented assumption that
read errors are independent, see below as to why they not
be.

> Have you ever encountered such problems, or are you
> using more robust hardware?

yes.  i have.  after unexpected power failure and apparently
a head crash, i have seen writes that appear to work but don't
followed by write failure and smart "threshold exceeded".
smart here isn't diagnostic but it allows one to rma a drive
without booting the drive wiz-bang tool.

- erik