Re: Should we be trying re-write on write errors?

["Keld Jørn Simonsen" <keld@dkuug.dk>]

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

On Sat, Nov 15, 2008 at 08:58:46AM +1100, Neil Brown wrote:
> On Friday November 14, greg@enjellic.com wrote:
> > 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?
> 
> 
> > 
> > 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 :-)
> 
> > 
> > Best wishes for a pleasant weekend.
> 
> And for you!
> 
> NeilBrown
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