diff --git a/Documentation/howto/dangling-objects.txt b/Documentation/howto/dangling-objects.txt new file mode 100644 index 0000000000..e82ddae3cf --- /dev/null +++ b/Documentation/howto/dangling-objects.txt @@ -0,0 +1,109 @@ +From: Linus Torvalds +Subject: Re: Question about fsck-objects output +Date: Thu, 25 Jan 2007 12:01:06 -0800 (PST) +Message-ID: +Archived-At: +Abstract: Linus describes what dangling objects are, when they + are left behind, and how to view their relationship with branch + heads in gitk + +On Thu, 25 Jan 2007, Larry Streepy wrote: + +> Sorry to ask such a basic question, but I can't quite decipher the output of +> fsck-objects. When I run it, I get this: +> +> git fsck-objects +> dangling commit 2213f6d4dd39ca8baebd0427723723e63208521b +> dangling commit f0d4e00196bd5ee54463e9ea7a0f0e8303da767f +> dangling blob 6a6d0b01b3e96d49a8f2c7addd4ef8c3bd1f5761 +> +> +> Even after a "repack -a -d" they still exist. The man page has a short +> explanation, but, at least for me, it wasn't fully enlightening. :-) +> +> The man page says that dangling commits could be "root" commits, but since my +> repo started as a clone of another repo, I don't see how I could have any root +> commits. Also, the page doesn't really describe what a dangling blob is. +> +> So, can someone explain what these artifacts are and if they are a problem +> that I should be worried about? + +The most common situation is that you've rebased a branch (or you have +pulled from somebody else who rebased a branch, like the "pu" branch in +the git.git archive itself). + +What happens is that the old head of the original branch still exists, as +does obviously everything it pointed to. The branch pointer itself just +doesn't, since you replaced it with another one. + +However, there are certainly other situations too that cause dangling +objects. For example, the "dangling blob" situation you have tends to be +because you did a "git add" of a file, but then, before you actually +committed it and made it part of the bigger picture, you changed something +else in that file and committed that *updated* thing - the old state that +you added originally ends up not being pointed to by any commit/tree, so +it's now a dangling blob object. + +Similarly, when the "recursive" merge strategy runs, and finds that there +are criss-cross merges and thus more than one merge base (which is fairly +unusual, but it does happen), it will generate one temporary midway tree +(or possibly even more, if you had lots of criss-crossing merges and +more than two merge bases) as a temporary internal merge base, and again, +those are real objects, but the end result will not end up pointing to +them, so they end up "dangling" in your repository. + +Generally, dangling objects aren't anything to worry about. They can even +be very useful: if you screw something up, the dangling objects can be how +you recover your old tree (say, you did a rebase, and realized that you +really didn't want to - you can look at what dangling objects you have, +and decide to reset your head to some old dangling state). + +For commits, the most useful thing to do with dangling objects tends to be +to do a simple + + gitk --not --all + +which means exactly what it sounds like: it says that you want to see the +commit history that is described by the dangling commit(s), but you do NOT +want to see the history that is described by all your branches and tags +(which are the things you normally reach). That basically shows you in a +nice way what the danglign commit was (and notice that it might not be +just one commit: we only report the "tip of the line" as being dangling, +but there might be a whole deep and complex commit history that has gotten +dropped - rebasing will do that). + +For blobs and trees, you can't do the same, but you can examine them. You +can just do + + git show + +to show what the contents of the blob were (or, for a tree, basically what +the "ls" for that directory was), and that may give you some idea of what +the operation was that left that dangling object. + +Usually, dangling blobs and trees aren't very interesting. They're almost +always the result of either being a half-way mergebase (the blob will +often even have the conflict markers from a merge in it, if you have had +conflicting merges that you fixed up by hand), or simply because you +interrupted a "git fetch" with ^C or something like that, leaving _some_ +of the new objects in the object database, but just dangling and useless. + +Anyway, once you are sure that you're not interested in any dangling +state, you can just prune all unreachable objects: + + git prune + +and they'll be gone. But you should only run "git prune" on a quiescent +repository - it's kind of like doing a filesystem fsck recovery: you don't +want to do that while the filesystem is mounted. + +(The same is true of "git-fsck-objects" itself, btw - but since +git-fsck-objects never actually *changes* the repository, it just reports +on what it found, git-fsck-objects itself is never "dangerous" to run. +Running it while somebody is actually changing the repository can cause +confusing and scary messages, but it won't actually do anything bad. In +contrast, running "git prune" while somebody is actively changing the +repository is a *BAD* idea). + + Linus +