linux-kernel - Re: [PATCH v3 13/16] Make generic_make_request handle arbitrarily large bios

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Message-ID: <20120525225852.GG5761@agk-dp.fab.redhat.com>
Date:	Fri, 25 May 2012 23:58:52 +0100
From:	Alasdair G Kergon <agk@...hat.com>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org,
	dm-devel@...hat.com, linux-fsdevel@...r.kernel.org, tj@...nel.org,
	axboe@...nel.dk, agk@...hat.com, neilb@...e.de,
	drbd-dev@...ts.linbit.com, bharrosh@...asas.com, vgoyal@...hat.com,
	mpatocka@...hat.com, sage@...dream.net, yehuda@...newdream.net
Subject: Re: [PATCH v3 13/16] Make generic_make_request handle arbitrarily
	large bios

On Fri, May 25, 2012 at 01:25:36PM -0700, Kent Overstreet wrote:
> But this approach becomes unwieldy and eventually breaks down with
> stacked devices and devices with dynamic limits, and it adds a lot of
> complexity. If the block layer could split bios as needed, we could

Complexity - yes - but if people didn't observe a genuine benefit, why
did they go to the trouble of writing this and getting it included?

> eliminate a lot of complexity elsewhere - particularly in stacked
> drivers. 

> Code that creates bios can then create whatever size bios are
> convenient, and more importantly stacked drivers don't have to deal with
> both their own bio size limitations and the limitations of the
> (potentially multiple) devices underneath them.

A theoretical argument.  Perhaps it's the right assessment of this
issue.  Perhaps it's not.  Or perhaps it depends on the use-case.

I made a theoretical argument from a different point of view in my last email.

I think a body of *empirical* evidence should provide the justification
for this particular change, and until such evidence is forthcoming we
should keep the status quo.

Alasdair

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