On Fri, Jan 23, 2015 at 11:27:39AM -0500, Vasily Tarasov wrote: [..] > > - Why did you implement an inline deduplication as opposed to out-of-line > > deduplication? Section 2 (Timeliness) in paper just mentioned > > out-of-line dedup but does not go into more details that why did you > > choose an in-line one. > > > > I am wondering that will it not make sense to first implement an > > out-of-line dedup and punt lot of cost to worker thread (which kick > > in only when storage is idle). That way even if don't get a high dedup > > ratio for a workload, inserting a dedup target in the stack will be less > > painful from performance point of view. > > Both in-line and off-line deduplication approaches have their own > pluses and minuses. Among the minuses of the off-line approach is > that it requires allocation of extra space to buffer non-deduplicated > writes, Well, that extra space requirement is temporary. So you got to pay the cost somewhere. Personally, I will be more than happy to consume more disk space when I am writing and not take a hit and let worker threads optimize space usage later. > re-reading the data from disk when deduplication happens (i.e. > more I/O used). Worker threads are supposed to kick in when disk is idle so it might not be as big a concern. > It also complicates space usage accounting and user > might run out of space though deduplication process will discover many > duplicated blocks later. Anyway, user needs to plan for extra space. De-dup is not exact science and one does not know how much will be the de-dup ratio in a data set. > > Our final goal is to support both approaches but for this code > submission we wanted to limit the amount of new code. In-line > deduplication is a core part, around which we can implement off-line > dedup by adding an extra thread that will reuse the same logic as > in-line deduplication. Ok. I am fine with building both if that makes sense. I also understand that there are pros/cons to both the approaches. Just that given the higt cost of inline dedupe, I am finding it little odd that it be implemented first as opposed to offline one. Anyway, I will spend some time on patches now. Thanks Vivek -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
