Re: Data Deduplication with the help of an online filesystem check

[Chris Mason <chris.mason@oracle.com>]

On Fri, Jun 05, 2009 at 02:20:48PM +0200, Tomasz Chmielewski wrote:
> Chris Mason wrote:
>> On Thu, Jun 04, 2009 at 10:49:19AM +0200, Thomas Glanzmann wrote:
>>> Hello Chris,
>>>
>>>>>  My question is now, how often can a block in btrfs be refferenced?
>>>> The exact answer depends on if we are referencing it from a single
>>>> file or from multiple files.  But either way it is roughly 2^32.
>>> could you please explain to me what underlying datastructure is used to
>>> monitor if the block is still referenced or already free? Is a counter
>>> used, bitmap (but that can't be if is 2^32) or some sort of list? I
>>> assume that a counter is used. If this is the case, I assume when a
>>> snapshot for example is deleted the reference counter of every block
>>> that was referenced in the snapshot will be decremented by one. Is this
>>> correct or am I missing something here?
>>
>> It is a counter and a back reference.  With Yan Zheng's new format work,
>> the limit is not 2^64.
>>
>> When a snapshot is deleted, the btree is walked to efficiently drop the
>> references on the blocks it referenced.
>>
>> From a dedup point of view, we'll want the dedup file to hold a
>> reference on the file extents.  The kernel ioctl side of things will
>> take care of that part.
>
> I wonder how well would deduplication work with defragmentation? One  
> excludes the other to some extent.

Very much so ;)  Ideally we end up doing dedup in large extents, but it
will definitely increase the overall fragmentation of the FS.

-chris