linux-kernel - Re: [PATCH 08/27] staging: lustre: avoid to use bio->bi

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Message-ID: <20160410144145.GA24062@infradead.org>
Date:	Sun, 10 Apr 2016 07:41:45 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	James Simmons <jsimmons@...radead.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Ming Lei <tom.leiming@...il.com>, Jens Axboe <axboe@...com>,
	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
	Boaz Harrosh <boaz@...xistor.com>,
	Oleg Drokin <oleg.drokin@...el.com>,
	Andreas Dilger <andreas.dilger@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"John L. Hammond" <john.hammond@...el.com>,
	Frank Zago <fzago@...y.com>,
	Mike Rapoport <mike.rapoport@...il.com>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Julia Lawall <Julia.Lawall@...6.fr>,
	Al Viro <viro@...iv.linux.org.uk>,
	"moderated list:STAGING - LUSTRE PARALLEL FILESYSTEM" 
	<lustre-devel@...ts.lustre.org>,
	"open list:STAGING SUBSYSTEM" <devel@...verdev.osuosl.org>
Subject: Re: [PATCH 08/27] staging: lustre: avoid to use bio->bi_vcnt directly

On Sun, Apr 10, 2016 at 03:37:42PM +0100, James Simmons wrote:
> 
> > The lloop driver should be removed entirely - use the loop driver
> > instead.
> 
> I talked with Andreas last week at our annual Lustre users group meeting 
> about this. The reason I was told for existance is that some users were
> using files on a Lustre file system with the loop back device. The 
> performance was really bad at the time so a lloop was developed to 
> overcome those limitations. Its been a long time so perhaps its time
> to look at the default loop driver again to see if can perform now. If
> it doesn't we will go the route of reworking the lloop driver in the
> spirit of the cryptoloop device.

The loop driver now supports using AIO/DIO on any file systems that
implements ->read_iter and ->write_iter. If lustre doesn't support
those or doesn't have proper performance using them it should be
addressed in the file system.

Note that the dio mode in the loop device is not the default and you
need to manually enabled it, keep that in mind when testing.