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

>
> On 12/7/2022 6:27 PM, Junio C Hamano wrote:
> > "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.
>
> I agree that this is a trade-off, and we should both be careful about
> whether or not we even make this a possibility for certain file
> formats. The index is an interesting case for a couple reasons:
>
> 1. Writes block users. Writing the index takes place in many user-
>    blocking foreground operations. The speed improvement directly
>    impacts their use. Other file formats are typically written in
>    the background (commit-graph, multi-pack-index) or are super-
>    critical to correctness (pack-files).
>
> 2. Index files are short lived. It is rare that a user leaves an
>    index for a long time with many staged changes. That's the condition
>    that's required for losing an index file to cause a loss of work
>    (or maybe I'm missing something). Outside of staged changes, the
>    index can be completely destroyed and rewritten with minimal impact
>    to the user.
>