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