Subject: Re: git only writing 4k at a time

From: "brian m. carlson" <[email-masked]>
Date: Wed, 18 Jun 2025 22:27:30 +0000

On 2025-06-18 at 20:58:52, Haylin Moore wrote:
> Hiya list,
> 
> I've been investigating some performance issues around git clones over
> network mounts. We have noticed that git is only writing 4k at a time.
> These small serial writes are making it such that even though each
> write is only a 3ms operation, the total time balloons. Looking around
> the source code I found that reftable_writer is initialized by default
> (though I cannot find the block_size argument being supplied in my
> cursory look) always to DEFAULT_BLOCK_SIZE (4096). Is there some way
> to increase/configure this block size such that larger writes happen?
> In git/Documentation/config/reftable.adoc this block size is mentioned
> in a manner that almost feels configurable, but I'm not sure if this
> is just internal for development.

It's fine to adjust reftable.blockSize upwards if you'd like, which
controls the block size for reftable writes (which is what you're seeing
if the writes are from the reftable code).  I think at least some
versions of JGit use 64 KiB for various reasons.

As the documentation describes, there may be some performance penalties
during reads since more refs will have to be read, so reading a single
ref will likely be more expensive.  However, you may find that
acceptable and you can adjust the values such that they provide the
right balance in your environment.  I would definitely recommend a
power-of-two block size, though.
-- 
brian m. carlson (they/them)
Toronto, Ontario, CA