Toby Thain wrote:
On 5-Apr-08, at 8:31 AM, Ric Wheeler wrote:
Toby Thain wrote:
On 4-Apr-08, at 2:58 PM, Ric Wheeler wrote:
Toby Thain wrote:
On 3-Apr-08, at 8:14 PM, Zan Lynx wrote:
On Tue, 2008-04-01 at 15:51 -0400, Jeff Mahoney wrote:
Ric's right about disk drives, though. They'll remap the bad sectors
automatically at the hardware level. When you start to see bad
sectors
at the file system level, it means that the sectors reserved for
remapping have been exhausted and you should replace the disk.
There are a couple of cases where you can see bad block errors on
a good
drive.
If a block is written with a bad CRC for some reason...the write head
got a freak blip or it lost power as it was writing, or the data went
corrupt while sitting on disk, then it will read as a bad block, but
rewriting would fix it.
A RAID media verify or a badblocks -n run can usually fix these.
Only if your RAID uses CRCs (most don't).
ZFS is the real answer to undetected corruption.
--Toby
Zan is right - even on a local drive, a write can repair some
sectors with bad protection bits. All disks have per sector data
protection (reed solomon encoding, etc) and there are lots of those
bits per sector.
That does not protect against writing bad data, only some errors
internal to drive. There is a long way to travel between CPU and
drive. Cable, controller, RAM, etc, etc, etc. ZFS protects the entire
data path.
--Toby
If you want to protect the entire data path, you are looking at
something like DIF which protects even more of the data path than ZFS
since it adds a check from application space to the IO stack ;-)
ZFS does not export its protection bits up the stack.
Correct, but it protects everything up to the system call. RAID does not
even get close, even with perfect error reporting (which doesn't really
exist anyway). :)
--Toby
When you look in detail at how data is lost in working systems, it is
always interesting to look at the big buckets of common failures and
make sure that we balance the complexity and cost (in money or in
performance) against the realized improvement.
What RAID does well is to protect against the leading and really, really
common error case which is single or few sector errors on a disk drive.
Those errors are almost always reported as IO errors and RAID systems
(including our MD software RAID) will do the right thing when only one
sector in a stripe is bad with a media error.
The interesting question is what failure is the second most common.
That, from what I see, is normally application/SW errors. It can be bugs
in the fs or IO stack, but also it is also common to lose data from bad
applications.
I don't have first hand experience with ZFS, but in any complicated
system you have a danger to increase the error rate (certainly for early
adopters ;-)) while the developers try to figure out how their
implementation differs from their pristine design (or what the design
concept missed).
My measured results of the reliability of reiserfs (v3) over a really
large population show that we do quite well (when you use barriers or
disable the write cache).
It will be interesting to look for the first ZFS study (like the CMU
paper by Bianca on disk failure, the google paper on failures and the
recent NetApp/UWisc papers on IO stack failures) to see how ZFS does in
the wild.
ric
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