linux-ext4 - Re: recursive mtime patches

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Date:	Thu, 14 Apr 2011 10:12:26 +0300
From:	Amir Goldstein <amir73il@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: recursive mtime patches

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.

I did not check how the "journal guided RAID resync" patches deal
with the same issue, but they store all modified data block numbers
in descriptor blocks, which is a lot more than just modified inodes.

>> 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.


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