Re: [patch 4/5] fail-injection capability for disk IO

[Jens Axboe <axboe@suse.de>]

On Wed, Aug 23 2006, Andrew Morton wrote:
> On Wed, 23 Aug 2006 14:03:55 +0200
> Jens Axboe <axboe@suse.de> wrote:
> 
> > On Wed, Aug 23 2006, Akinobu Mita wrote:
> > > This patch provides fail-injection capability for disk IO.
> > > 
> > > Boot option:
> > > 
> > > 	fail_make_request=<probability>,<interval>,<times>,<space>
> > > 
> > > 	<probability>
> > > 
> > > 		specifies how often it should fail in percent.
> > > 
> > > 	<interval>
> > > 
> > > 		specifies the interval of failures.
> > > 
> > > 	<times>
> > > 
> > > 		specifies how many times failures may happen at most.
> > > 
> > > 	<space>
> > > 
> > > 		specifies the size of free space where disk IO can be issued
> > > 		safely in bytes.
> > > 
> > > Example:
> > > 
> > > 	fail_make_request=100,10,-1,0
> > > 
> > > generic_make_request() fails once per 10 times.
> > 
> > Hmm dunno, seems a pretty useless feature to me.
> 
> We need it.  What is the FS/VFS/VM behaviour in the presence of IO
> errors?  Nobody knows, because we rarely test it.  Those few times where
> people _do_ test it (the hard way), bad things tend to happen.  reiserfs
> (for example) likes to go wobble, wobble, wobble, BUG.

You misunderstood me - a global parameter is useless, as it makes it
pretty impossible for people to use this for any sort of testing (unless
it's very specialized). I didn't say a feature to test io errors was
useless!

> > Wouldn't it make a lot
> > more sense to do this per-queue instead of a global entity?
> 
> Yes, I think so.  /sys/block/sda/sda2/make-it-fail.

Precisely.

-- 
Jens Axboe