linux-kernel - Re: [PATCH] Allow SysRq emergency sync to thaw frozen filesystems

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Date:	Sat, 17 Jan 2009 09:44:14 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	7eggert@....de
CC:	Eric Sandeen <sandeen@...deen.net>, Valdis.Kletnieks@...edu,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	t-sato@...jp.nec.com
Subject: Re: [PATCH] Allow SysRq emergency sync to thaw frozen filesystems

Bodo Eggert wrote:
> Eric Sandeen <sandeen@...deen.net> wrote:
>> Valdis.Kletnieks@...edu wrote:
>>> On Thu, 15 Jan 2009 21:59:10 CST, Eric Sandeen said:
> 
>>>> Oh, actually, I'd think not.  If the freeze was done properly by the
>>>> filesystem, all data was flushed, the fs was quiesced, and new IO was
>>>> blocked.  pdflush should never be visiting these...
>>> Yes, but a lot of 'if's - and usually you're reaching for sysrq-S precisely
>>> *because* you suspect that stuff wasn't happening properly on its own...
>> Actually, only one if - if the fs implemented freeze properly.
>>
>> Well, the use case I envision here is something like:
>>
>> # freeze /my/mount/point/to/fs/to/snapshot
>>
>> except oops, that wasn't mounted, and you just froze your root fs.
> 
> Maybe freeze should protect against that by requiring to specify the exact
> mountpount, unless you say freeze --subdir?

That's a good idea.  My "freeze" above was a hypothetical tool which
doesn't really exist yet, but should get that enhancement.  :)
(xfs_freeze does not do this checking today, it probably should)

-Eric
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