Re: btrfs on disk stability

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

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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.

   Hugo.

-- 
Hugo Mills             | Q: What goes, "Pieces of seven! Pieces of seven!"?
hugo@... carfax.org.uk | A: A parroty error.
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