On Wed, 2015-10-07 at 20:31 +0200, Michael Haggerty wrote: > On 10/07/2015 03:51 AM, David Turner wrote: > > On Mon, 2015-10-05 at 17:47 +0200, Michael Haggerty wrote: > >> On 09/29/2015 12:02 AM, David Turner wrote: > >>> Add a database backend for refs using LMDB. [...] > >> > >> I think you have said before that if one writer holds the write lock on > >> the DB, then other writers fail immediately. Is that correct? If so, is > >> that something that can be adjusted? I think it would be preferable for > >> the second writer to retry acquiring the write lock for a little while > >> with a timeout (as we now do when trying to acquire the packed-refs > >> lock). Otherwise you could have the unhappy situation that somebody > >> spends a long time pushing a packfile to a server, only to have the > >> reference update be rejected at the last moment due to a lock conflict > >> with another process that was touching completely different references. > >> We already do before/after consistency checks when updating references, > >> so you wouldn't even have to add such code in the backend yourself. > > > > No, the second writer waits for the first writer to unlock (or for it to > > crash). > > Cool, that's better behavior. > > > [...] > >> Do you store "peeled" reference values for tags, as is done in > >> packed-refs? I think that is an important optimization. > > > > No. Do you happen to know in what situations this is a performance > > benefit, so that I can benchmark? I suspect it would matter much less > > for the LMDB backend, because lookups are pretty quick. > > The reference lookup speed is not relevant here. "Peeling" is applied to > references that point at tag objects (a.k.a. annotated tags). It means > that the tag object is looked up to see what *it* points at (recursively > if necessary) and the result is stored to the packed-refs file in a > specially-formatted extra line that looks like > > 17f9f635c101aef03874e1de1d8d0322187494b3 refs/tags/v2.6.0 > ^be08dee9738eaaa0423885ed189c2b6ad8368cf0 > > I think a good command to benchmark would be `git show-refs -d` in a > repository with a number of annotated tags. This command's output is > similar to the output of `git ls-remote <remote>` and also comes up > during reference negotiation when fetching (so its performance is > definitely not moot). > > > [...] > >> Currently we discard the reflog for a reference when the reference is > >> deleted. [...] > >> Have you thought about removing this limitation in the lbdb backend? > > > > I'm going for feature parity first. We can always add new functionality > > later. This particular function would be pretty straightforward to add, > > I think. > > +1 > > > [...] > >>> +The rsync and file:// transports don't work yet, because they > >>> +don't use the refs API. > >> > >> Do they fail gracefully? > > > > Not particularly gracefully. > > > > rsync: link_stat "/home/dturner/git/t/trash > > directory.t5510-fetch/.git/packed-refs" failed: No such file or > > directory (2) > > rsync error: some files/attrs were not transferred (see previous errors) > > (code 23) at main.c(1183) [sender=3.1.1] > > fatal: Could not run rsync to get refs > > ------------- > > > > The problem is that rsync on the client assumes that packed-refs exists. > > We could hack it to also check for refdb. > > I guess this is something that will have to be improved sooner or later, > though I guess not as a precondition for merging this patch series. > > > [...] > >> I'm somewhat surprised that you only register the lmdb backend if it is > >> used in the main repo. I would expect the backend to be registered > >> unconditionally on startup. The cost is trivial, isn't it? > > > > Yeah, but this was the easiest place to do it. > > OK. > > > [...] > > I'm really happy about your work. > > Regarding strategy: I think a good approach would be to get as much of > the preparatory work as possible (the abstraction and separation of > refs-be-files) to the point where it can be merged before there is too > much more code churn in the area. That work is not very controversial, I > think, and letting it wait for a long time will increase the likelihood > of conflicts with other people's changes. The refs-be-lmdb patches, on > the other hand, (1) will take longer to get polished, (2) will take > longer to review because other people are not familiar with LDMB, and > (3) won't bitrot very fast anyway because they don't overlap as much > with areas that other people are likely to work on. So I would advocate > working on those at a more deliberate pace and planning for them to be > merged as a separate batch.
Works for me. Would you like me to start sending those as a separate series, or shall I keep it as one series and let you split it as you choose? -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
