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[dm-devel] Re: [PATCH 3/3] Add timeout feature

On Wed, 9 Jul 2008 17:13:46 +1000
Dave Chinner <david fromorbit com> wrote:

> > one can argue about the need of doing the first 3 steps via a
> > userland loop; it sure sounds like one needs to be really careful
> > to not do any writes to the fs from the app that does snapshots
> > (and that includes doing any syscalls in the kernel that allocate
> > memory.. just because that already could cause unrelated data to be
> > written from inside the app. Not fun.)
> Bloody hell! Doesn't *anyone* understand that a frozen filesystem is
> *clean*? That the process of freezing it ensures all dirty data and
> metadata is written out before the freeze completes? And that once
> frozen, it can't be dirtied until unfrozen?
> That's 3 (or is it 4 - maybe 5 now that I think about it) different
> ppl in 24 hours that have made this same broken argument about
> being unable to write back dirty data on a frozen filesystem......

unless you also freeze the system as a whole (in a 'refrigerator
suspend' way).. the "clean" part is just about a nanosecond long. After
that stuff gets dirty again (you're doing that sendfile to receive more
packets from the FTP upload etc etc).
Sure you can pause those. But there's a real risk that you end up
pausing the app that you want to unfreeze the fs (via the memory
allocation->direct reclaim path). And no, mlock doesn't help.
Especially with delayed allocation, where data writes will cause
metadata activity, this is not just theory.

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