linux-kernel - Re: document ext3 requirements

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Message-ID: <20090104223346.GB1913@elf.ucw.cz>
Date:	Sun, 4 Jan 2009 23:33:46 +0100
From:	Pavel Machek <pavel@...e.cz>
To:	Theodore Tso <tytso@....edu>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org
Subject: Re: document ext3 requirements

Hi!

On Sat 2009-01-03 21:32:11, Theodore Tso wrote:
> On Sat, Jan 03, 2009 at 01:38:15PM +0100, Pavel Machek wrote:
> > +Requirements
> > +============
> > +
> > +Ext3 expects disk/storage subsystem to behave sanely. On sanely
> > +behaving disk subsystem, data that have been successfully synced will
> > +stay on the disk. Sane means:
> > +
> > +* writes to media never fail. Even if disk returns error condition during
> > +  write, ext3 can't handle that correctly, because success on fsync was already
> > +  returned when data hit the journal.
> > +
> > +	   (Fortunately writes failing are very uncommon on disks, as they
> > +	   have spare sectors they use when write fails.)
> 
> This is not unique to ext3; per the discussion two weeks ago, this is
> largely because of the fsync() interface not possibly being able to

Ok, so I guess I should split the patch to truly ext3-specific part,
and the part that is common for all the filesystems. I guess I'll need
some help with everything but ext2 and ext3...

> return errors caused by failures when creating or modifying parent
> directories.  Given this, it's a bit misleading to place this in the
> Documentation/filesystems/ext3.txt.  At the minimum it should include
> a discussion about what the issues might be, and given that pretty
> much any Unix/Linux filesystem doesn't have a way of reflecting these
> errors to application programs, it probably should be in a
> filesystem-independent documentation file.

Ok. I'll have to think about good name of that file.

> > +* either write caching is disabled, or hw can do barriers and they are enabled.
> > +
> > +	   (Note that barriers are disabled by default, use "barrier=1"
> > +	   mount option after making sure hw can support them). 
> 
> We really should get akpm to agree to accept the patch to default
> barriers by default instead.  :-)

:-). Yes, that would help a bit.

(No, it is not complete solution. barrier=0/writeback on should be
still documented as unsafe).
									Pavel

-- 
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