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SubjectRe: Problem with I/O counts in ll_rw_blk.c?
Hi,

On Mon, 25 Oct 1999 14:53:22 +0100 (BST), Malcolm Beattie
<mbeattie@sable.ox.ac.uk> said:

> The problem is that the only requests logged for activity on an ext2
> filesystem (1K block size) are ones with nr_sectors of 2 and the
> number of spacing of those requests show that each request is actually
> carrying much more data (typically 128 sectors) which is somehow
> merged into the request but not reflected in the nr_sectors count.

It depends very much on where in the request lifetime you measure the
nr_sectors, because as you've found, the merging code can adjust things
before the request ever gets as far as the driver.

> Looking at various block drivers, I can't find anything relying on
> nr_sectors so maybe driver authors already implicitly "know"
> this. Indeed, floppy.c has a buffer_chain_size function that walks the
> buffer head list of the current request to work out the number of
> adjacent sectors.

The floppy.c code is looking for sectors which are adjacent _in memory_,
so that it can merge them into a single DMA request. Nothing more.

--Stephen

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