On Thu, 21 Jun 2007, Mattias Wadenstein wrote:
On Thu, 21 Jun 2007, Neil Brown wrote:
I have that - apparently naive - idea that drives use strong checksum,
and will never return bad data, only good data or an error. If this
isn't right, then it would really help to understand what the cause of
other failures are before working out how to handle them....
In theory, that's how storage should work. In practice, silent data
corruption does happen. If not from the disks themselves, somewhere along the
path of cables, controllers, drivers, buses, etc. If you add in fcal, you'll
get even more sources of failure, but usually you can avoid SANs (if you care
about your data).
Well, here is a couple of the issues that I've seen myself:
A hw-raid controller returning every 64th bit as 0, no matter what's on disk.
With no error condition at all. (I've also heard from a collegue about this
on every 64k, but not seen that myself.)
An fcal switch occasionally resetting, garbling the blocks in transit with
random data. Lost a few TB of user data that way.
Add to this the random driver breakage that happens now and then. I've also
had a few broken filesystems due to in-memory corruption due to bad ram, not
sure there is much hope of fixing that though.
Also, this presentation is pretty worrying on the frequency of silent data
corruption:
https://indico.desy.de/contributionDisplay.py?contribId=65&sessionId=42&confId=257
/Mattias Wadenstein
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Very interesting slides/presentation, going to watch it shortly.
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