Re: btrfs on disk stability

[Hugo Mills <hugo@carfax.org.uk>]

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On Tue, May 26, 2015 at 09:29:25AM +0000, sri wrote:
> Hugo Mills <hugo <at> carfax.org.uk> writes:
> 
> > 
> > On Tue, May 26, 2015 at 08:45:06AM +0000, sri wrote:
> > > Hi,
> > > 
> > > According to btrfs wiki page, under "Stability status" it is written 
> that
> > > 
> > > "The filesystem disk format is no longer unstable".
> > > 
> > > Does this mean if there are more I/Os are going on a btrfs file 
> system, 
> > > copy of entire disk (all disk blocks) gives a stable disk?
> > 
> >    No, it means that the format isn't changing in incompatible ways
> > any more. You're guaranteed that if you upgrade your kernel, the FS
> > will still be readable on the new kernel. (And, if you don't enable
> > any extra features with btrfstune, that the kernel will still be
> > readable if you downgrade to the earlier kernel you were using).
> > 
> > > Just to elaborate more, if btrfs file system is created on 2 disks 
> > > /dev/sda and /dev/sdb and if I start copying blocks of sda and sdb 
> to sdc 
> > > and sdc respectively by just opening file handlers of sda and sdb 
> and 
> > > mounting the new copy via /dev/sdc and /dev/sdd will give consistent 
> file 
> > > system??
> > 
> >    That's always the case, with the very large caveat that you remove
> > /dev/sdc and /dev/sdd from the system before you try mounting anything
> > related to that FS. Making block level copies of btrfs filesystems and
> > leaving them visible to the same kernel as the originals is a very bad
> > idea, and can cause massive FS corruption.
> > 
> >    It's OK to make the copy, but not to try mounting the FS with both
> > copies present, as the kernel will see both copies as the same
> > filesystem (because they have the same UUID), and it will get very
> > confused about which device(s) it's meant to be writing to.
> > 
> 
>   Thank you Hugo,
>   copies of disk will not be there once block level copy is done. Assume 
> that sdc and sdd (which are used for copy sda and sdb) are ISCSI disks 
> from different machine and as soon as disk copy sda->sdc and sdb->sdd is 
> done, I will remove both disks and use it on other machine.
> 
> In this case 
> 1) After copy at disk level whether the btrfs on sdc and sdd will be in 
> consistent state? even though there are any IOs on original disks sda 
> and sdb?
>   My guess is no but I am not sure

   If you're trying to copy a filesystem that's in use, then no, you
can't expect the copies to be consistent.

   Imagine: At the point where you've copied half the disk, and the FS
makes two updates which depend on each other -- one in the piece
you've already copied, and one in the piece you haven't copied
yet. Also, by this point, you've got the situation I warned against
above: multiple copies of a filesystem that's mounted, so your FS is
probably already damaged beyond repair. Doing it with a read-only
mount of the FS might be possible.

   Basically, you can only do this reliably if the filesystem is not
mounted.

> 2) new disks sdc and sdd can be mounted on another machine where btrfs 
> fs is supported ?  

   Yes.

   Hugo.

-- 
Hugo Mills             | Do not meddle in the affairs of wizards, for they
hugo@... carfax.org.uk | are subtle, and quick to anger.
http://carfax.org.uk/  |
PGP: E2AB1DE4          |                                        J.R.R. Tolkein

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