[Tux3] Generic filesystem freeze

[Michael Keulkeul]

Hi

Yes it does fail to guarantee consistency between filesystem
and application state, but it's a tool to guarantee it :

if you want to take a consistent snapshot of you database :
1- put your DB in backup mode, or make our app layer happy
2- freeze your FS
3- sync everything
4- take a snapshot (of the block devices)
5- unfreeze FS
6- DB back in standard mode


If you skip the freeze step, disk write still can happen between end of 3
and end of 4 so you can get dirty ondisk images. As discussed in a earlier
thread, tux3 design should enable to "tux3freeze" a filesystem (get a clean
ondisk image) without blocking writes (just throttle N io/sec until buffers
P% full, then return something and unfreeze).

And this would enable us to do something like this :


Repeat
  1- put your DB in backup mode, or freeze app io/s...
  2- tux3freeze your FS
  3- sync everything (maybe not necessary)
  4- DB or app back in standard mode
  5- take a snapshot (of the block devices)
Until my fs still tux3freezed at that moment
(not unfreeze as we go back to normal state when we're full)






On 1/12/09, Daniel Phillips <phillips at phunq.net> wrote:
>
> On Sunday 11 January 2009 23:57, Michael Keulkeul wrote:
> > Hi all
> >
> > I don't know if you already know, but generic filesystem freeze interface
> is
> > being add to linux kernel, check linux 2.6.29-rc1 changelog for more
> info.
> >
> > Best regards
> >
> > Michael B.
>
> We will certainly support it, but note my skepticism about the value
> of it.  It certainly fails to gaurantee consistency between filesystem
> and application state, and it does introduce deadlock risk, so I am not
> sure what it is for.  Please enlighten me :-)
>
> Regards,
>
> Daniel
>
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