On 22.10.2018 22:14, Evgeny Kotkov wrote: > Branko Čibej <br...@apache.org> writes: > >> Still missing is a mechanism for the libsvn_wc (and possibly >> libsvn_client) to determine the capabilities of the working copy at >> runtime (this will be needed for deciding whether to use compressed >> pristines). > FWIW, I tried the idea of using LZ4 to compress the pristines and storing > small > pristines as blobs in the `PRISTINE` table. I was particularly interested in > how such change would affect the performance and what kind of obstacles > would have to be dealt with.
Nice! I did some simpler tests by compressing exported trees, but this is definitely better. > In the attachment you will find a more or less functional implementation of > this idea that might be useful to some extent. The patch is a proof of > concept: it doesn't include the WC compatibility bits and most certainly > doesn't have everything necessary in place. But in the meanwhile, I think > that is might give a good approximation of what can be expected from the > approach. > > The patch applies to the `better-pristines` branch. > > A couple of observations: > > - As expected, the combined size of the pristines is halved when the data > itself is compressible, thus making the working copy 25% smaller. Yes, that was my observation as well. In fact, though, storing small BLOBs in the database itself should have even better effects, since the space on disk actually used by a file is rounded up to the nearest cluster size, but SQLite's blocks are typically much smaller than that. > - A variety of the callers currently access the pristine contents by reading > the corresponding files. That doesn't work in case of compressed pristines > or pristines stored as BLOBs. > > I think that ideally we would want to use streams as much as possible, and > only spill the uncompressed pristine contents to temporary files when we > need to pass them to external tools, etc.; and that temporary files need > to be backed by a work queue to avoid leaving them in place in case of an > application crash. Yes and yes. Keeping those temporary spilled files on disk could turn out to be a problem, finding a reasonable time to delete them without having to run cleanup will be rather important, I think. > The patch does that kind of plumbing to some extent, but that part of the > work is not complete. The starting point is around wc_db_pristine.c: > svn_wc__db_pristine_get_path(). > > - Using BLOBs to store the pristine contents didn't have a measurable impact > on the speed of the WC operations such as checkout in my experiments on > Windows. These experiments were not comprehensive, and also I didn't run > the tests on *nix. I wouldn't expect much change in performance but would expect better use of the disk, as explained above. > - There's also the deprecated svn_wc_get_pristine_copy_path() public API that > would require plumbing to maintain compatibility; the patch performs it by > spilling the pristine contents result into a temporary file whose lifetime > is attached to the `result_pool`. Ack; that's one reasonable definition of "lifetime." But I suspect that any users of that function expect the pristine file to survive at least to the next WC cleanup. > (I probably won't be able to continue the work on this patch in the nearby > future; posting this in case it might be useful.) Thanks, it definitely is useful! -- Brane
