On Tue, Nov 16 2021, Neeraj Singh wrote: > On Tue, Nov 16, 2021 at 12:10 AM Ævar Arnfjörð Bjarmason > <avarab@xxxxxxxxx> wrote: >> >> >> On Mon, Nov 15 2021, Neeraj K. Singh via GitGitGadget wrote: >> >> > * Per [2], I'm leaving the fsyncObjectFiles configuration as is with >> > 'true', 'false', and 'batch'. This makes using old and new versions of >> > git with 'batch' mode a little trickier, but hopefully people will >> > generally be moving forward in versions. >> > >> > [1] See >> > https://lore.kernel.org/git/pull.1067.git.1635287730.gitgitgadget@xxxxxxxxx/ >> > [2] https://lore.kernel.org/git/xmqqh7cimuxt.fsf@gitster.g/ >> >> I really think leaving that in-place is just being unnecessarily >> cavalier. There's a lot of mixed-version environments where git is >> deployed in, and we almost never break the configuration in this way (I >> think in the past always by mistake). > >> In this case it's easy to avoid it, and coming up with a less narrow >> config model[1] seems like a good idea in any case to unify the various >> outstanding work in this area. >> >> More generally on this series, per the thread ending in [2] I really > > My primary goal in all of these changes is to move git-for-windows over to > a default of batch fsync so that it can get closer to other platforms > in performance > of 'git add' while still retaining the same level of data integrity. > I'm hoping that > most end-users are just sticking to defaults here. > > I'm happy to change the configuration schema again if there's a > consensus from the Git > community that backwards-compatibility of the configuration is > actually important to someone. > > Also, if we're doing a deeper rethink of the fsync configuration (as > prompted by this work and > Eric Wong's and Patrick Steinhardts work), do we want to retain a mode > where we fsync some > parts of the persistent repo data but not others? If we add fsyncing > of the index in addition to the refs, > I believe we would have covered all of the critical data structures > that would be needed to find the > data that a user has added to the repo if they complete a series of > git commands and then experience > a system crash. Just talking about it is how we'll find consensus, maybe you & Junio would like to keep it as-is. I don't see why we'd expose this bad edge case in configuration handling to users when it's entirely avoidable, and we're still in the design phase. >> don't get why we have code like this: >> >> @@ -503,10 +504,12 @@ static void unpack_all(void) >> if (!quiet) >> progress = start_progress(_("Unpacking objects"), nr_objects); >> CALLOC_ARRAY(obj_list, nr_objects); >> + plug_bulk_checkin(); >> for (i = 0; i < nr_objects; i++) { >> unpack_one(i); >> display_progress(progress, i + 1); >> } >> + unplug_bulk_checkin(); >> stop_progress(&progress); >> >> if (delta_list) >> >> As opposed to doing an fsync on the last object we're >> processing. I.e. why do we need the step of intentionally making the >> objects unavailable in the tmp-objdir, and creating a "cookie" file to >> sync at the start/end, as opposed to fsyncing on the last file (which >> we're writing out anyway). >> >> 1. https://lore.kernel.org/git/211110.86r1bogg27.gmgdl@xxxxxxxxxxxxxxxxxxx/ >> 2. https://lore.kernel.org/git/20211111000349.GA703@neerajsi-x1.localdomain/ > > It's important to not expose an object's final name until its contents > have been fsynced > to disk. We want to ensure that wherever we crash, we won't have a > loose object that > Git may later try to open where the filename doesn't match the content > hash. I believe it's > okay for a given OID to be missing, since a later command could > recreate it, but an object > with a wrong hash looks like it would persist until we do a git-fsck. Yes, we handle that rather badly, as I mentioned in some other threads, but not doing the fsync on the last object v.s. a "cookie" file right afterwards seems like a hail-mary at best, no? > I thought about figuring out how to sync the last object rather than some random > "cookie" file, but it wasn't clear to me how I'd figure out which > object is actually last > from library code in a way that doesn't burden each command with > somehow figuring > out its last object and communicating that. The 'cookie' approach > seems to lead to a cleaner > interface for callers. The above quoted code is looping through nr_objects isn't it? Can't a "do fsync" be passed down to unpack_one() when we process the last loose object?
