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
There is work on adding DIF (data integrity f?) which is extra
bytes that arrays or local drives can store for application level
protection. Martin Petersen has some good slides about this on linux:
http://oss.oracle.com/projects/data-integrity/documentation/
ZFS, for example, or more specifically its lvm layer, could use DIF
to add this kind of protection.
The other way to go is to use an enterprise class array - they all
have multiple layers of data integrity baked in to deal with and
correct these kind of errors.
ric
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