Re: Readahead with softraid1

[Jens Axboe <axboe@suse.de>]

On Fri, Jul 08 2005, Erik Slagter wrote:
> On Fri, 2005-07-08 at 08:16 -0400, Danny Cox wrote:
> 
> > > What am I doing wrong here???
> > 
> > 	Nothing.  I'll take a shot at answering this one instead of lurking
> > this time.  Then, I'll crawl back under my rock.
> > 
> > 	The raid1 driver keeps a "last visited block" for each drive.  This is
> > the block number that was most recently read or written by that drive.
> > When a read request arrives, the driver examines each drive for the
> > nearest last visited block to the one requested.  Guess what?  If the
> > read starts with drive sda, then it will *always* be the one chosen to
> > service the read in the future, because the last visited block number is
> > only one off.  This would only change if there are multiple processes
> > performing I/O on the md device.  Then, it may switch to another drive.
> > In any case, it will *tend* to stick with the same drive.
> > 
> > 	Did I explain that well, or only muddy the waters?
> 
> perfect explanation, thanks (and also Jens!).
> 
> Is this a design decision or is it fundamentaly impossible to split the
> work amongst several drives?  I guess the md driver at least do a
> prefetch of the next block/chunk on the "other" drive(s)?

Not sure if it's a design decision or just "this works ok, I'll fix it
later". Clearly there is a lot of room for improvement in the balancing
logic, to get more cases correct/faster. It's quite doable to split a
bio and send bits of it to various drives.

-- 
Jens Axboe