On Sat, Apr 19, 2014 at 12:43 AM, Junio C Hamano <gits...@pobox.com> wrote: > Having said that, I do not think there is a fundamental reason why > the stat data has to live inside the same index file. A separate > file is just fine, as long as you can reliably detect that they went > out of sync for whatever reason (e.g. "the index proper updated, a > stale stat file left beind"), and storing the trailer checksum from > the corresponding index in this new file is an obvious and good > solution.
I've gone further and store index updates (including entry removals and additions) to the second index file so that index I/O cost is now proportional to the number of changed entries, not the work tree size (sort of). Which makes it scale much better when the work tree is huge. There is one flaw though. I'm expecting many "yuck" responses from people. So let's try to settle it now, or drop the idea. The idea is we can support another mode, where index content is stored in two files, the small $GIT_DIR/index and large $GIT_DIR/index.base. "index" contains changes that should be applied to "index.base". Whenever you do something to the index, "index" records those actions. Git reads both index.base and index, then replay the action to have the final index in memory. "index.base" contains full worktree data and remains unchanged until "index" becomes too big/slow that changes should be merged back to "index.base". This works great (my prototype passed the test suite), and even greater than index v5 because v5 still rewrites the whole index file when an entry is added or removed. But there is a problem with atomic update. The good old rename() does not work well with 2 files. This is not a problem with the C part, I can still make atomic update work. Scripts, on the other hand, may rely on "mv" or similar commands/functions to prepare a temp index and move it to $GIT_DIR/index. The workaround is merge back two files into a single index file so that scripts can "mv $temp_index" as before and pay the whole-index I/O penalty. An alternative is store two files in one, the one index file actually consists two subfiles. We avoid the atomic update problem, but we pay I/O cost for writing 10MB every time an index is updated (but not hashing 10MB file) and introduce a new index format. This is even yuckier in my opinion. Should I continue, or drop it? -- Duy -- 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
