linux-ext4 - Re: recursive mtime patches

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Message-ID: <20110414092117.GB5054@quack.suse.cz>
Date:	Thu, 14 Apr 2011 11:21:17 +0200
From:	Jan Kara <jack@...e.cz>
To:	Amir Goldstein <amir73il@...il.com>
Cc:	Jan Kara <jack@...e.cz>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: recursive mtime patches

On Thu 14-04-11 10:12:26, Amir Goldstein wrote:
> On Thu, Apr 14, 2011 at 12:39 AM, Jan Kara <jack@...e.cz> wrote:
> > On Wed 13-04-11 21:16:40, Amir Goldstein wrote:
> >> On Tue, Apr 12, 2011 at 5:48 PM, Jan Kara <jack@...e.cz> wrote:
> >> > modification stamps have possibly larger race windows but I haven't really
> >> > tried how much (I just know that even mtime races are not that hard to
> >> > trigger if you try). So it really depends on how big reliability do you
> >> > expect and I personally don't find much value in just rescanning and
> >> > checking for mtime after a crash. Reading all the data and doing checksum
> >> > certainly has more value but at a high cost.
> >> >
> >>
> >> What do you thing about the approach to store recursively modified dir inodes in
> >> a journal "modified inode descriptor block" and update the recursive mtime of
> >> those dirs on journal recovery?
> >  The trouble is you don't know the number of directories that may need
> > to have timestamp updated - you find that out only as you travel upwards.
> > So it's hard to reserve any fixed space for this.
> >
> 
> True, but you can save *so* many inode numbers in just one descriptor
> block and in case of an overflow, we can just pass a hint to the top
> level application to do a full directory scan, so I hardly see that as a
> big problem.
  Well, about 1000 but you can still have about 8000 inodes modified in a
transaction for a standard 128 MB journal. You can notify the userspace
when an overflow happens but the interface gets kind of ugly... Also it
would be only specific to ext3/4 while I'd prefer to get a wider fs
support.

> >> I would also consider to use a mount option rec_mtime and then just
> >> store recursive
> >> mtime in the directory's inode mtime instead of an extended attribute.
> >> That doesn't break any contract with user space, it's just a re-interpretation
> >> of the dir modification notion.
> >  It breaks POSIX specification - POSIX pretty much specifies when mtime is
> > supposed to be changed - so I'm not sure we really want to do that...
> 
> I disagree, POSIX doesn't forbid a user space daemon from touching directory
> inodes and updating their mtime. The rec_mtime feature should be treated as
> a little kernel "daemon" which propagates information to user space by touching
> recursively modified directories.
  OK, if you look at it this way it makes some sense. You loose the
distinction whether something has been created / deleted in the directory
or whether only something happened in its subdirectory or file but that
does not seem too important for any use case I can think of.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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