Re: Snapper snapshot comparison algorithm - send/receive questions

[Arvin Schnell <aschnell@suse.de>]

On Sat, Dec 01, 2012 at 01:24:20PM +0530, nafisa mandliwala wrote:

> I needed help with understanding the snapshot comparison algorithm
> that snapper uses and its shortcomings. From reading the code, what I
> understood is that it does a block by block compare. I'm not very sure
> if that's the best way to go about it. Also, since the send receive
> code is still in development stages, is there a scope to add more
> functionality to it?

Mainly snapper does a directory traversal and a block-by-block
comparison of files which is indeed a inefficient comparison
algorithm.

I already had a look at using the new send/receive ioctl to
improve the comparison and have a few question:

1. snapper only needs to know that a file has changed but the
   send stream also contains the new content which might has to
   be read from disk.

   Mark Fasheh has made a patch for a flag for the send ioctl to
   not include the content (don't know if he posted it here since
   I was kicked of the list twice). With no write commands in the
   stream how can I detect a file content change? Currently there
   seems to be a truncate after the write but is that guaranteed?

   Btw: There are many apparently useless truncates, e.g. after a
   chmod. What are these good for?

2. Is it possible to add a ioctl for send that takes open
   file-descriptors for parent_root and clone_sources? Otherwise
   it's insecure to use from snapper (which has root privileges
   and sometimes operates in directories users can modify). Such
   a ioctl would also reduce the number of btrfs related
   functions needed.

3. Overall lots of functions are needed to use the
   send/receive. Are there any plans to create a btrfs-library
   that contains the required functions
   e.g. btrfs_read_and_process_send_stream, subvol_uuid_search
   and tree_search?

Regards,
  Arvin

-- 
Arvin Schnell, <aschnell@suse.de>
Senior Software Engineer, Research & Development
SUSE LINUX Products GmbH, GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer, HRB 16746 (AG Nürnberg)
Maxfeldstraße 5
90409 Nürnberg
Germany