Re: [PATCH v3 6/8] maintenance: add incremental-repack task

From: Derrick Stolee <stolee@gmail.com>
To: Jonathan Tan <jonathantanmy@google.com>, gitgitgadget@gmail.com
Cc: git@vger.kernel.org, sandals@crustytoothpaste.net,
	steadmon@google.com, jrnieder@gmail.com, peff@peff.net,
	congdanhqx@gmail.com, phillip.wood123@gmail.com,
	emilyshaffer@google.com, sluongng@gmail.com,
	derrickstolee@github.com, dstolee@microsoft.com
Subject: Re: [PATCH v3 6/8] maintenance: add incremental-repack task
Date: Thu, 24 Sep 2020 10:05:17 -0400	[thread overview]
Message-ID: <691ac9f0-0678-8c95-18ad-7b54f92ae531@gmail.com> (raw)
In-Reply-To: <20200922232654.914862-1-jonathantanmy@google.com>

On 9/22/2020 7:26 PM, Jonathan Tan wrote:
>> +incremental-repack::
>> +	The `incremental-repack` job repacks the object directory
>> +	using the `multi-pack-index` feature. In order to prevent race
>> +	conditions with concurrent Git commands, it follows a two-step
>> +	process.
> 
> [snip]
> 
>> First, it deletes any pack-files included in the
>> +	`multi-pack-index` where none of the objects in the
>> +	`multi-pack-index` reference those pack-files; this only happens
>> +	if all objects in the pack-file are also stored in a newer
>> +	pack-file. Second, it selects a group of pack-files whose "expected
>> +	size" is below the batch size until the group has total expected
>> +	size at least the batch size; see the `--batch-size` option for
>> +	the `repack` subcommand in linkgit:git-multi-pack-index[1]. The
>> +	default batch-size is zero, which is a special case that attempts
>> +	to repack all pack-files into a single pack-file.
> 
> This lacks the detail of what happens to the selected group of packfiles
> (in the second step) - in particular, that a new packfile is created and
> the MIDX is rewritten so that all references to the selected group are
> updated to refer to the new packfile instead, thus making it possible to
> delete the selected group of packfiles in a subsequent first step. All
> this is explained in the documentation of git-multi-pack-index (expire
> and repack), though, so it might be better to refer to that. E.g.

Here is my attempt to incorporate your recommendations into this doc:

incremental-repack::
	The `incremental-repack` job repacks the object directory
	using the `multi-pack-index` feature. In order to prevent race
	conditions with concurrent Git commands, it follows a two-step
	process. First, it calls `git multi-pack-index expire` to delete
	pack-files unreferenced by the `multi-pack-index` file. Second, it
	calls `git multi-pack-index repack` to select several small
	pack-files and repack them into a bigger one, and then update the
	`multi-pack-index` entries that refer to the small pack-files to
	refer to the new pack-file. This prepares those small pack-files
	for deletion upon the next run of `git multi-pack-index expire`.
	The selection of the small pack-files is such that the expected
	size of the big pack-file is at least the batch size; see the
	`--batch-size` option for the `repack` subcommand in
	linkgit:git-multi-pack-index[1]. The default batch-size is zero,
	which is a special case that attempts to repack all pack-files
	into a single pack-file.

> Do we need get_midx_filename() to be global?

No, that does not appear to be important (to this patch). It _was_
necessary when doing the "verify, then delete if problematic" mode.
Thanks for catching it now that it is not necessary.

Thanks,
-Stolee