linux-ext4 - Re: [PATCH 3/3] Add timeout feature

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Message-ID: <20080714063617.GF31949@elf.ucw.cz>
Date:	Mon, 14 Jul 2008 08:36:17 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	jim owens <jowens@...com>
Cc:	linux-fsdevel@...r.kernel.org, Dave Chinner <david@...morbit.com>,
	Theodore Tso <tytso@....edu>,
	Arjan van de Ven <arjan@...radead.org>,
	Miklos Szeredi <miklos@...redi.hu>, hch@...radead.org,
	t-sato@...jp.nec.com, akpm@...ux-foundation.org,
	viro@...IV.linux.org.uk, linux-ext4@...r.kernel.org,
	xfs@....sgi.com, dm-devel@...hat.com, linux-kernel@...r.kernel.org,
	axboe@...nel.dk, mtk.manpages@...glemail.com
Subject: Re: [PATCH 3/3] Add timeout feature

On Sun 2008-07-13 13:15:43, jim owens wrote:
> Pavel Machek wrote:
>
>>> This means ONLY SOME metadata (or no metadata) is flushed and
>>> then all metadata updates are stopped.  User/kernel writes
>>> to already allocated file pages WILL go to a frozen disk.
>>
>> That's the difference here. They do write file data, and thus avoid
>> mmap()-writes problem.
>>
>> ...and they _still_ provide auto-thaw.
>> 								Pavel
>
> One of the hardest things to make people understand is that
> stopping file data writes in the filesystem during a freeze
> is not just dangerous, it is also __worthless__ unless you
> have a complete "user environment freeze" mechanism.

Eh?

> And unless each independent component has a freeze and they
> can all be coordinated, the data in the pipeline is never
> stable enough to say "if you stop all writes to disk and
> take a snapshot, this is the same as an orderly shutdown,
> backup, restore, and startup".

If you also freeze data writes, at least snapshot is not worse than
_unexpected_ shutdown.

And apps should already be ready for unexpected shutdowns (using fsync
etc).

								Pavel

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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