Mozilla has added tens of thousands of files to the Firefox repo in the past few months and we have plans to add tens of thousands more shortly. Working directory update times (especially in automation, which has to do fresh checkouts with a somewhat high frequency since we rely on ephemeral compute instances) were borderline tolerable before. With the addition of tens of thousands of new files, working directory updates are starting to put noticeable strain on systems.
Mozilla can make due with sparse checkouts - we don't yet have so much repo data that we need narrow clone, although narrow would be useful. Facebook's sparse checkout extension has existed for years and my understanding is it gets the job done. When I asked at the sprint why sparse isn't part of the core distribution, someone mentioned it is because sparse and narrow have different, competing concepts for defining a sparse/narrow profile and these will need to be reconciled before either can be accepted into core. Is there a timeline for unifying the profiles and adding sparse to the core distribution? Also, if I had to use sparse as it is now, what recommendations would you have? Should I create an /.hgsparse in the repo? Or should I perhaps "hide" the file elsewhere and use `hg sparse --import-rules`? Keep in mind that Mozilla doesn't have as tightly managed setup as other companies do. Once we support sparse in any way, we have to think about BC and what happens when e.g. old versions of Mercurial/sparse are used on a new revision.
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