linux-ext4 - Re: recursive mtime patches

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Message-ID: <BANLkTinO7G1YxZCP1-PXG5Nvyas=QdAunQ@mail.gmail.com>
Date:	Wed, 13 Apr 2011 21:16:40 +0200
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 Tue, Apr 12, 2011 at 5:48 PM, Jan Kara <jack@...e.cz> wrote:
>  Hi,
>
> On Mon 11-04-11 16:37:57, Amir Goldstein wrote:
>> Do you have an uptodate version of your recursive mtime patches?
>> The only version I can find online is the original series from 2007.
>  I've put latest version (against 2.6.37) to
> http://beta.suse.com/private/jack/recursive_mtime/
>

Thanks, I'll see if we can use it for our application.

>> I am interested in the patches for indexdb-like application,
>> so persistence after crash is also important for my use case.
>> Your patches would require the application to perform a full
>> directory scan after crash, right?
>  OK, it depends. Currently, even mtime updates are not reliable (data can
> be written to a file while mtime update is not yet committed).  Recursive

Yes, good point.

> 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?

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.

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