Re: btrfs-image and btrfs send related queries

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

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On Fri, Apr 15, 2016 at 12:41:36PM +0000, sri wrote:
> Hi,
> 
> I have couple of queries related to btrfs-image, btrfs send and with 
> combination of two.
> 1)
> I would like to know if a btrfs source file system is spread across more 
> than 1 disks, does btrfs-image require same number of disks to create 
> empty file system without files content??

   I don't _think_ you need as many devices as there were originally.

> 2) would btrfs-image can be modified to keep only given subvolume foot 
> print and related meta data to bring back file system live on destination 
> disk?
> 
>    To elaborate more on this, Lets say I have 5 subvolumes on source btrfs 
> and i run btrfs-image written to destination disk say /dev/sdd. In this 
> process, can btrfs-image modified to just have only 1 subvolume and skipp 
> other 4 subvolumes and write to destination i.. /dev/sdd so that when I 
> mount /dev/sdd , I will have btrfs with only 1 subvolume with no data.

   For a first approximation, you could just drop any FS tree from the
image which wasn't the target one. After that, it turns into a
complicated accounting exercise to drop all of the back-refs to the
missing FS trees, and to drop all the extent records for the
non-shared data and the metadata for the missing FS trees.

   It's probably going to be complicated, and will basically involve
rewriting most of the image to avoid the metadata you didn't want.

> 3) If 3 can successful, can btrfs-image further changed to include data of 
> selected subvolume which gives files data also written to /dev/sdd which 
> would be kind of a backup of a subvolume taken out of a btrfs file system 
> which is having more than 1 subvolumes.

   If you're going to do all the hard work of (2), then (3) is a
reasonable logical(?) extension.

   On the other hand, what's wrong with simply using send/receive? It
gives you a data structure (a FAR-format send stream) which contains
everything you need to reconstruct a subvolume on a btrfs different
to the original.

   Hugo.

-- 
Hugo Mills             | Mary had a little lamb
hugo@... carfax.org.uk | You've heard this tale before
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