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

[Eric Sandeen <sandeen@sandeen.net>]

Valdis.Kletnieks@vt.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.

I was thinking more recovery from admin error, not programming error...

If we're using sysrq to work around any possible programming error, then
we have a pretty tough job to make sure that it always works, no?

-Eric