Btrfs devel - Jan 2013 - About Chunk Tree recover

Hi, everyone.

About 1 years ago, we implemented the chunk tree recover function,
but it has not been applied till now because that implementation
need change the disk format.
(http://marc.info/?l=linux-btrfs&m=129914269932543&w=2
 http://marc.info/?l=linux-btrfs&m=130976668006281&w=2
 http://marc.info/?l=linux-btrfs&m=129914269932543&w=2)

Recently, I reconsidered the implementation of this function, and
found a new approach that needn''t change the disk format. That is
the external chunk tree backup, just like external journal device
of ext4. The basic idea is:

- specify a external file or device which is used to backup the
  chunk tree when mount.
- When mount, compare the super block in the external file/device
  with the super block of the btrfs, if the checksum of the super
  block in the externel file/device is right, and the FS UUID and
  generation are the same as the fs, it means the chunk tree in
  the external file/device is valid, needn''t rebuild it. Otherwise,
  we will rebuild the chunk tree in the external file/device
  according to the chunk tree of the fs.
- When we allocate a new chunk, we will log the new chunk
  information into the external file/device
- sync the external file/device when committing the transaction
- If the chunk tree of the fs is corrupted, we use the information
  in the external file/device to recover it.
  
By this way, we needn''t change disk format and also needn''t do
a
block device scan which need lots of time, and is very hard to
find the start address and length of a chunk.

Any comment for this idea?

Thanks
Miao
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On Tue, Jan 29, 2013 at 04:13:47AM -0700, Miao Xie
wrote:
> Hi, everyone.
> 
> About 1 years ago, we implemented the chunk tree recover function,
> but it has not been applied till now because that implementation
> need change the disk format.
> (http://marc.info/?l=linux-btrfs&m=129914269932543&w=2
>  http://marc.info/?l=linux-btrfs&m=130976668006281&w=2
>  http://marc.info/?l=linux-btrfs&m=129914269932543&w=2)
> 
> Recently, I reconsidered the implementation of this function, and
> found a new approach that needn''t change the disk format. That is
> the external chunk tree backup, just like external journal device
> of ext4. The basic idea is:
I do like the idea of a dedicated chunk backup area, outside the
filesystem.  But, I think we need to be able to fall back to the
scanning operation.

The chunk tree backup actually fits well into the log area I''m setting
up for raid5/6.  The log area is really just a dedicated chunk where
I''m
stuffing blocks to avoid read/modify/write and to make sub stripe writes
power cut safe.

-chris
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