Re: Software RAID when it works and when it doesn't

[Alberto Alonso <alberto@ggsys.net>]

On Thu, 2007-10-18 at 17:26 +0200, Goswin von Brederlow wrote:
> Mike Accetta <maccetta@laurelnetworks.com> writes:

> What I would like to see is a timeout driven fallback mechanism. If
> one mirror does not return the requested data within a certain time
> (say 1 second) then the request should be duplicated on the other
> mirror. If the first mirror later unchokes then it remains in the
> raid, if it fails it gets removed. But (at least reads) should not
> have to wait for that process.
> 
> Even better would be if some write delay could also be used. The still
> working mirror would get an increase in its serial (so on reboot you
> know one disk is newer). If the choking mirror unchokes then it can
> write back all the delayed data and also increase its serial to
> match. Otherwise it gets really failed. But you might have to use
> bitmaps for this or the cache size would limit its usefullnes.
> 
> MfG
>         Goswin

I think a timeout on both: reads and writes is a must. Basically I
believe that all problems that I've encountered issues using software
raid would have been resolved by using a timeout within the md code.

This will keep a server from crashing/hanging when the underlying 
driver doesn't properly handle hard drive problems. MD can be 
smarter than the "dumb" drivers.

Just my thoughts though, as I've never got an answer as to whether or
not md can implement its own timeouts.

Alberto