On 22/09/2012 7:49 p.m., Arne Jansen wrote:

On 09/22/12 05:38, Jp Wise wrote:

quoted

Good morning, I'm working on an offline deduplication script intended to
work around the copy-on-write functionality of BTRFS.

Simply put - is there any existing utility to compare two files (or
dirs) and output if the files share the same physical extents / data
blocks on disk?
- aka - they're CoW copies.

I'm not actively working with BTRFS yet, but for the project i'm working
on it's looking to the be most suitable candidate, and the CoW
functionality avoids issues with file changes that hardlinks would create.
 From reading other posts, aware the information could be pulled out via
btrfs-debug-tree, but it would then involve parsing the entire output to
locate the required files inodes and their extents which seems like
quite a roundabout way to retrieve the information.

Also my programming skills aren't  up to the task of trying to pull the
tree data directly from the filesystem to do it, and I'd like to avoid
doing byte-by-byte comparisons on all files as it's inefficient if the
file can instead be identified as a CoW copy.

The information is available in the kernel, but to find a good way to
extract it you have to describe in much more detail what you intend to
do. What I, first of all, don't understand, is, why you need the
information of already shared (=deduped) blocks to build a dedup. Don't
you want to find data that is identical, but not shared, instead?

Hi Arne, that's exactly my issue. I want to filter out files that have 
already been de-duped to avoid re-checking two files that already share 
the same data blocks.
  In this usecase, I can identify potential duplicates via filename data 
(this dataset also stores a basic checksum as part of the filename), but 
rather than then doing a secondary checksum/cmp, I'd like to instead 
check if it's already sharing the same data blocks (ie: already 
de-duped). If two files share the same data blocks, I can safely say 
it's already de-duped and move onto the next potential match with doing 
the additional crunching of checksum/cmp to verify the match.

Logically there should also be usecases where someone has made a data 
copy in the past and may be uncertain if they made a reflink copy or a 
full copy and wants to recheck if they share the same data blocks.

quoted

Open to suggestions of other tools that could be used to acheive the
desired result.

Afaik without playing with it myself fiemap can give you information
about the mappings of each file. If the mappings of 2 files match,
the data is shared.

OK, have just done some searching on fiemap and located a code example 
using it to pull the file extent data.
http://smackerelofopinion.blogspot.com/2010/01/using-fiemap-ioctl-to-get-file-extents.html
Will have a play around to see if i might be able to hack it up to 
compare two files, or just parse it's output between two files to 
identify matches.  Thank you for the pointer. :)

Likewise if anyone else knows of an existing utility to do a 
non-bytewise compare between two files, and just check if they share the 
same datablocks please let me know. :)

quoted

Thanks.
Jp.
-- 
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html