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Re: [PATCH 3/3] Add timeout feature

To: jim owens <jowens@xxxxxx>
Subject: Re: [PATCH 3/3] Add timeout feature
From: Pavel Machek <pavel@xxxxxxx>
Date: Mon, 14 Jul 2008 08:36:17 +0200
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, Dave Chinner <david@xxxxxxxxxxxxx>, Theodore Tso <tytso@xxxxxxx>, Arjan van de Ven <arjan@xxxxxxxxxxxxx>, Miklos Szeredi <miklos@xxxxxxxxxx>, hch@xxxxxxxxxxxxx, t-sato@xxxxxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, viro@xxxxxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, dm-devel@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, axboe@xxxxxxxxx, mtk.manpages@xxxxxxxxxxxxxx
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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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