Arthur Britto wrote:
On Tue, 2008-06-24 at 08:58 +1000, Neil Brown wrote:
On Sunday June 22, ahbritto@xxxxxxx wrote:
smartmontools (http://smartmontools.sourceforge.net/) can be configured
to passively scan hard drives for defects in the background. The block
numbers of pending unreadable sectors are logged via syslog. These
sectors will be remapped when written too.
It would be great if this worked end to end with linux software raid to
automatically repair the bad sector.
Well, you can just get md to do a scan (echo check >
/sys/block/mdXX/md/sync_action) and it will find any read errors and
correct them.
True. However, a SMART on disk check requires no main board
resources. Some drives, when idle, may do background checking anyway.
This would provide a way to correct the error without needing to scan
the whole volume and other components with an md check. Error checking
may be less intrusive (vs retries to the exclusion of other work) than
normal for an attempted sector read. At least manufactures have the
option to give priority to actual read requests over background defect
checking.
This is almost always the case with disk arrays for example.
Extracting numbers from syslog is a fairly messy thing to try to do.
Maybe if smartmontools could report these in some other way -
e.g. run a program giving device and block number, we could write a
script that feeds that info to md.
We would need to map the device+offset to partition+offset, then find
out if that is a member of an md array, then request a limited-range
'check', which I think is possible with current code...
Do you know if smartmontools can provide this info in a more
controlled way?
I was thinking, a non-smartmontools specific method would be best. That
is: (1) some way for the md driver to request notification about pending
uncorrected read errors from a region of a block device and (2) some way
for a trusted application to inform the kernel about pending uncorrected
read errors (e.g. echo "start-stop > /sys/...").
-Arthur
One thing that you can do that is much less invasive is to use the "read
verify" command to scan the platter of the disk at a fairly low rate.
Read verify does not transfer data from the disk to the host and you can
issue fairly large requests (say 1MB at a time) as a background task per
drive. What you will get out of this is a validation that nothing has
failed at the disk sector level (i.e., each sector is still readable).
On detection of an error, you can go back and try to pin point the
failed sector with small IO's and then try to repair the damage with a
write (say from the other mirror in a RAID1 device).
This is useful, but is not an end to end data integrity check (like
Martin's T10 DIF work that was posted).
In general, it would also be really neat to figure out and API which
would let a higher level application (or file system) inform the block
level of an error and possibly ask for a read from another mirror.
ric
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