Greg Freemyer wrote:

quoted

On Sat, Nov 15, 2008 at 08:58:46AM +1100, Neil Brown wrote:
    

quoted

On Friday November 14, greg@enjellic.com wrote:
      

quoted

Hi Neil, hope the week is ending well for you and the rest of the
denizens on the linux-raid list.

Somewhat of a Gedanken question for you.

We currently attempt a re-write on read error for volumes which have
redundancy, ie. RAID[156] etc, on the bet that we can force a bad
sector remap.  Should we be attempting that (or do we) on a write
error as well?
        

I don't think so.
By the time md/raid gets an error status, lower levels (Whether driver
or firmware) should have retried as much as in appropriate.  Doing
further retries at the md level should be pointless.

For reads, we do retry.  But the purpose is to find out exactly which
block failed so that we can just re-write that block.  There is no
expectation that a block which previously failed a read will now
succeed.

Similarly there is no reason to expect that a block which previously
failed a write will now succeed.

I suggest that you might like to discuss your particular case with the
author of the driver for the device.  Maybe the driver should be
retrying.  Maybe the firmware is doing the wrong thing.

After all, you wouldn't expect every different filesystem to retry all
failed writes, would you?


      

quoted

BTW much thanks for the existing re-write code.  Countless mornings
I have said 'gee that Neil Brown was clever' when I see that one of
our machines cleaned up a potential problem before it became a bigger
one.
        

:-)
To be honest, that code was largely because people kept complaining
about read errors being too fatal and wanted something done.  The only
way to stop the flood of complaints was to fix something :-)

      

quoted

Best wishes for a pleasant weekend.
        

And for you!

NeilBrown
      

<<Moved from the top post to a bottom post>>

On Fri, Nov 14, 2008 at 7:47 PM, Keld Jørn Simonsen [off-list ref] wrote:
  

quoted

I would like to write something about this fo the wiki.
What exactly is done, and it is general for all of linux md raid?

best regards
keld

    

If you are going to document this in a wiki, please document when a
write error can occur because I totally don't understand how this one
occurred.

I thought they could only occur:

1) With bad media on the platter and the reallocatable sectors section
was already 100% utilized

2) Due to a CRC error on the comm path.  (flacky cable / power / etc.)

As I read the below errors, neither of those occurred.  And as Neil
said I believe the retrys related to CRC errors should be handled
below the MD level.

Greg
  

Most of the common write errors you see should not be retried, but you 
might see some writes fail due to transient conditions.

One possible condition would be vibrations, for example as you wheel a 
rack around in your data center or you bang into the computer.

 If you are using a SAN, you might also have transient link errors that 
will go away once the switch rights itself or someone plugs back in a 
new cable...

Ric

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