Re: Readahead with softraid1

[Erik Slagter <erik@slagter.name>]

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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)?

Now I am still wondering about the readahead issue. What would be a sane
setting? I guess having both the drives and md doing readahead is not
optimal?

Also I noticed that the default readahead value changed significantly
from 2.4 to 2.6, is there a particular reason for that?

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