linux-kernel - Re: [PATCH 1/4] scatterlist: new helper functions

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Date:	Tue, 15 Mar 2011 21:18:14 -0700 (PDT)
From:	Alex Dubov <oakad@...oo.com>
To:	akpm@...ux-foundation.org,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	maximlevitsky@...il.com, fujita.tomonori@....ntt.co.jp,
	James.Bottomley@...senPartnership.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] scatterlist: new helper functions

> 
> I don't think so.
> 
> Splitting (or merging) a request and playing with sg lists
> inside a
> driver is a bad idea. Such should be done in the block
> layer.
> 
> I still don't see why the block layer can't do that for the
> driver.
> 
> I believe that the helper functions for such should not be
> added.
> 

In a particular case of flash-like devices, letting the block layer
split requests into memory sequential blocks too often results in
unnecessary fragmentation of writes/erases.

If only one sg entry is requested from the block layer, it will be (more
often than not) only 1 or 2 pages in length, even if total size of
prospective write request spans multiple erase blocks.

So there are really only two options for legacy memorystick driver:
1. Play with scatterlists explicitly.
2. Make it an MTD backend, rather then stand-alone block device.

The second option makes more sense, but it is not necessarily the optimal
approach for implementation of this particular media format.

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