linux-kernel - Re: [PATCH 00/14] Pramfs: Persistent and protected ram filesystem

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Date:	Mon, 15 Jun 2009 08:51:28 -0700
From:	Bryan Henderson <hbryan@...ibm.com>
To:	Artem Bityutskiy <dedekind1@...il.com>
Cc:	Daniel Walker <dwalker@....ucsc.edu>,
	Jamie Lokier <jamie@...reable.org>,
	Linux Embedded <linux-embedded@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Marco <marco.stornelli@...il.com>
Subject: Re: [PATCH 00/14] Pramfs: Persistent and protected ram filesystem

> Marco wrote:
> >    To enable direct
> >    I/O at all times for all regular files requires either that
> >    applications be modified to include the O_DIRECT flag on all file
> >    opens, or that a new filesystem be used that always performs direct
> >    I/O by default."
> 
> This could be done as well by just introducing a "direct_io_only"
> mount option to a file-system which would need this feature.

A mount option would not be the right way.  Mount options are for things 
that are characteristic of the way you're going to access the files. 
_This_ is a characteristic of the block device.  So if one were to make 
this memory accessible with a block device, it would make more sense to 
have a block device ioctl.  And it wouldn't ask the question, "should I 
use direct I/O only," but "does this device have the performance 
characteristics of a classic disk drive?"

But it's possible that there's just no advantage to having a block device 
in the stack here.  When unix block devices were invented, their main 
purpose was that they could reorder reads and writes and do buffering and 
caching -- all things essential for disk drives.  We don't want to stretch 
the concept too far.

--
Bryan Henderson                     IBM Almaden Research Center
San Jose CA                         Storage Systems

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