Re: Poor read performance when sequential write presents

[Andrew Morton]

Jens Axboe wrote:
> 
> ...
> > But in 2.5, head-activeness went away and as far as I know, IDE and SCSI are
> > treated the same.  Odd.
> 
> It didn't really go away, it just gets handled automatically now.
> elv_next_request() marks the request as started, in which case the i/o
> scheduler won't consider it for merging etc. SCSI removes the request
> directly after it has been marked started, while IDE leaves it on the
> queue until it completes. For IDE TCQ, the behaviour is the same as with
> SCSI.

It won't consider the active request at the head of the queue for 
merging (making the request larger).  But it _could_ consider the
request when making decisions about insertion (adding a new request
at the head of the queue because it's close-on-disk to the active
one).   Does it do that?

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