Re: [PATCH 0/4] Optionally skip hashing index on write

From: Junio C Hamano <gitster@pobox.com>
Date: Dec 7, 2022, 23:27 UTC
Message-ID: <xmqqilim6bhh.fsf@gitster.g>
In-Reply-To: <pull.1439.git.1670433958.gitgitgadget@gmail.com>

"Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:

> Writing the index is a critical action that takes place in multiple Git
> commands. The recent performance improvements available with the sparse
> index show how often the I/O costs around the index can affect different Git
> commands, although reading the index takes place more often than a write.

The sparse-index work is great in that it offers correctness while taking advantage of the knowledge of which part of the tree is quiescent and unused to boost performance. I am not sure a change to reduce file safety can be compared with it, in that one is pure improvement, while the other is trade-off.

As long as we will keep the "create into a new file, write it fully and fsync + rename to the final" pattern, we do not need the trailing checksum to protect us from a truncated output due to index-writing process dying in the middle, so I do not mind that trade-off, though.

Protecting files from bit flipping filesystem corruption is a different matter. Folks at hosting sites like GitHub would know how often they detect object corruption (I presume they do not have to deal with the index file on the server end that often, but loose and pack object files have the trailing checksums the same way) thanks to the trailing checksum, and what the consequences are if we lost that safety (I am guessing it would be minimum, though).

Thanks.