Re: [PATCH v2 0/3] refs: sync loose refs to disk before committing them

> On Wed, Nov 10, 2021 at 12:40:50PM +0100, Patrick Steinhardt wrote:
> 
> > Please note that I didn't yet add any performance numbers or tests.
> > Performance tests didn't show any conclusive results on my machine given
> > that I couldn't observe any noticeable impact at all, and I didn't write
> > tests yet given that I first wanted to get some feedback on this series.
> > If we agree that this is the correct way to go forward I'll of course
> > put in some more time to add them.
> 
> Here's a mild adjustment of the quick test I showed earlier in [1]. It
> uses your version for all cases, but swaps between the three config
> options, and it uses --atomic to put everything into a single
> transaction:
> 
>   $ hyperfine -L v false,true,batch -p ./setup 'git -C dst.git config core.fsyncreffiles {v}; git.compile push --atomic dst.git refs/heads/*'
>   Benchmark 1: git -C dst.git config core.fsyncreffiles false; git.compile push --atomic dst.git refs/heads/*
>     Time (mean ± σ):      10.5 ms ±   0.2 ms    [User: 7.8 ms, System: 3.9 ms]
>     Range (min … max):    10.1 ms …  11.0 ms    108 runs
>    
>   Benchmark 2: git -C dst.git config core.fsyncreffiles true; git.compile push --atomic dst.git refs/heads/*
>     Time (mean ± σ):     402.5 ms ±   6.2 ms    [User: 13.9 ms, System: 10.7 ms]
>     Range (min … max):   387.3 ms … 408.9 ms    10 runs
>    
>   Benchmark 3: git -C dst.git config core.fsyncreffiles batch; git.compile push --atomic dst.git refs/heads/*
>     Time (mean ± σ):      21.4 ms ±   0.6 ms    [User: 8.0 ms, System: 5.2 ms]
>     Range (min … max):    20.7 ms …  24.9 ms    99 runs
>    
>   Summary
>     'git -C dst.git config core.fsyncreffiles false; git.compile push --atomic dst.git refs/heads/*' ran
>       2.05 ± 0.07 times faster than 'git -C dst.git config core.fsyncreffiles batch; git.compile push --atomic dst.git refs/heads/*'
>      38.51 ± 1.06 times faster than 'git -C dst.git config core.fsyncreffiles true; git.compile push --atomic dst.git refs/heads/*'
> 
> So yes, the "batch" case is much improved. It's still more expensive
> than skipping the fsync entirely, but it's within reason. The trick,
> though is the --atomic. If I drop that flag, then "batch" takes as long
> as "true". And of course that's the default.

> I wonder if that means we'd want an extra mode: do the WRITEOUT_ONLY
> flush, but _don't_ do a HARDWARE_FLUSH for each transaction. That
> doesn't give you durability, but it (in theory) solves the out-of-order
> journal problem without paying any performance cost.
> 
> I say "in theory" because I'm just assuming that the WRITEOUT thing
> works as advertised. I don't have an easy way to test the outcome on a
> power failure at the right moment here.
> 
> -Peff
> 
> [1] https://lore.kernel.org/git/YYTaiIlEKxHRVdCy@coredump.intra.peff.net/