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From: Stefan Beller <sbeller@google.com>
To: Derrick Stolee <stolee@gmail.com>
Cc: git <git@vger.kernel.org>, Derrick Stolee <dstolee@microsoft.com>,
	Ævar Arnfjörð Bjarmason <avarab@gmail.com>,
	Jonathan Nieder <jrnieder@gmail.com>,
	Jonathan Tan <jonathantanmy@google.com>,
	Martin Fick <mfick@codeaurora.org>
Subject: Re: [PATCH 02/23] midx: add midx format details to pack-format.txt
Date: Mon, 11 Jun 2018 12:19:13 -0700
Message-ID: <CAGZ79kaDiXnAuRym3HHQPEZh9z0-sPL5V4Yur5-_RojsyLJsYA@mail.gmail.com> (raw)
In-Reply-To: <20180607140338.32440-3-dstolee@microsoft.com>

Hi Derrick,
On Thu, Jun 7, 2018 at 7:03 AM Derrick Stolee <stolee@gmail.com> wrote:
>
> The multi-pack-index (MIDX) feature generalizes the existing pack-
> index (IDX) feature by indexing objects across multiple pack-files.
>
> Describe the basic file format, using a 12-byte header followed by
> a lookup table for a list of "chunks" which will be described later.
> The file ends with a footer containing a checksum using the hash
> algorithm.
>
> The header allows later versions to create breaking changes by
> advancing the version number. We can also change the hash algorithm
> using a different version value.
>
> We will add the individual chunk format information as we introduce
> the code that writes that information.
>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  Documentation/technical/pack-format.txt | 49 +++++++++++++++++++++++++
>  1 file changed, 49 insertions(+)
>
> diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
> index 70a99fd142..17666b4bfc 100644
> --- a/Documentation/technical/pack-format.txt
> +++ b/Documentation/technical/pack-format.txt
> @@ -252,3 +252,52 @@ Pack file entry: <+
>      corresponding packfile.
>
>      20-byte SHA-1-checksum of all of the above.
> +
> +== midx-*.midx files have the following format:
> +
> +The meta-index files refer to multiple pack-files and loose objects.

So is it meta or multi?

> +In order to allow extensions that add extra data to the MIDX, we organize
> +the body into "chunks" and provide a lookup table at the beginning of the
> +body. The header includes certain length values, such as the number of packs,
> +the number of base MIDX files, hash lengths and types.
> +
> +All 4-byte numbers are in network order.
> +
> +HEADER:
> +
> +       4-byte signature:
> +           The signature is: {'M', 'I', 'D', 'X'}
> +
> +       1-byte version number:
> +           Git only writes or recognizes version 1
> +
> +       1-byte Object Id Version
> +           Git only writes or recognizes verion 1 (SHA-1)

s/verion/version/

> +       1-byte number (C) of "chunks"
> +
> +       1-byte number (I) of base multi-pack-index files:
> +           This value is currently always zero.

Oh? Are meta-index and multi-index files different things?

> +       4-byte number (P) of pack files
> +
> +CHUNK LOOKUP:
> +
> +       (C + 1) * 12 bytes providing the chunk offsets:
> +           First 4 bytes describe chunk id. Value 0 is a terminating label.
> +           Other 8 bytes provide offset in current file for chunk to start.
> +           (Chunks are provided in file-order, so you can infer the length
> +           using the next chunk position if necessary.)

It is so nice to have the header also have 12 bytes, so it fits right into the
lookup table. So an alternative point of view:

  If a chunk needs to store more than 8 bytes, we'll have an offset after
  the first 4 bytes that describe the chunk, otherwise you can store the 8 bytes
  of information directly after the 4 bytes.
   "MIDX" is a special chunk and must come first (does it?) and only once
  as it contains the version number.

> +       The remaining data in the body is described one chunk at a time, and
> +       these chunks may be given in any order. Chunks are required unless
> +       otherwise specified.
> +
> +CHUNK DATA:
> +
> +       (This section intentionally left incomplete.)
> +
> +TRAILER:
> +
> +       H-byte HASH-checksum of all of the above.

This means we have to rehash the whole file for updating its contents.
okay.

  reply index

Thread overview: 192+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-07 14:03 [PATCH 00/23] Multi-pack-index (MIDX) Derrick Stolee
2018-06-07 14:03 ` [PATCH 01/23] midx: add design document Derrick Stolee
2018-06-11 19:04   ` Stefan Beller
2018-06-18 18:48     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 02/23] midx: add midx format details to pack-format.txt Derrick Stolee
2018-06-11 19:19   ` Stefan Beller [this message]
2018-06-18 19:01     ` Derrick Stolee
2018-06-18 19:41       ` Stefan Beller
2018-06-07 14:03 ` [PATCH 03/23] midx: add midx builtin Derrick Stolee
2018-06-07 17:20   ` Duy Nguyen
2018-06-18 19:23     ` Derrick Stolee
2018-06-11 21:02   ` Stefan Beller
2018-06-18 19:40     ` Derrick Stolee
2018-06-18 19:55       ` Stefan Beller
2018-06-18 19:58         ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 04/23] midx: add 'write' subcommand and basic wiring Derrick Stolee
2018-06-07 17:27   ` Duy Nguyen
2018-06-07 14:03 ` [PATCH 05/23] midx: write header information to lockfile Derrick Stolee
2018-06-07 17:35   ` Duy Nguyen
2018-06-12 15:00   ` Duy Nguyen
2018-06-19 12:54     ` Derrick Stolee
2018-06-19 14:59       ` Duy Nguyen
2018-06-19 15:24         ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 06/23] midx: struct midxed_git and 'read' subcommand Derrick Stolee
2018-06-07 17:54   ` Duy Nguyen
2018-06-20 13:13     ` Derrick Stolee
2018-06-07 18:31   ` Duy Nguyen
2018-06-20 13:33     ` Derrick Stolee
2018-06-20 15:07       ` Duy Nguyen
2018-06-20 16:39         ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 07/23] midx: expand test data Derrick Stolee
2018-06-07 14:03 ` [PATCH 08/23] midx: read packfiles from pack directory Derrick Stolee
2018-06-07 18:03   ` Duy Nguyen
2018-06-20 16:33     ` [PATCH] packfile: generalize pack directory list Derrick Stolee
2018-06-07 14:03 ` [PATCH 09/23] midx: write pack names in chunk Derrick Stolee
2018-06-07 18:26   ` Duy Nguyen
2018-06-21 15:25     ` Derrick Stolee
2018-06-21 17:38       ` Junio C Hamano
2018-06-22 18:25         ` Derrick Stolee
2018-06-22 18:31           ` Junio C Hamano
2018-06-22 18:32             ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 10/23] midx: write a lookup into the pack names chunk Derrick Stolee
2018-06-09 16:43   ` Duy Nguyen
2018-06-21 17:23     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 11/23] midx: sort and deduplicate objects from packfiles Derrick Stolee
2018-06-09 17:07   ` Duy Nguyen
2018-06-21 17:54     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 12/23] midx: write object ids in a chunk Derrick Stolee
2018-06-09 17:25   ` Duy Nguyen
2018-06-07 14:03 ` [PATCH 13/23] midx: write object id fanout chunk Derrick Stolee
2018-06-09 17:28   ` Duy Nguyen
2018-06-21 19:49     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 14/23] midx: write object offsets Derrick Stolee
2018-06-09 17:41   ` Duy Nguyen
2018-06-07 14:03 ` [PATCH 15/23] midx: create core.midx config setting Derrick Stolee
2018-06-07 14:03 ` [PATCH 16/23] midx: prepare midxed_git struct Derrick Stolee
2018-06-09 17:47   ` Duy Nguyen
2018-06-07 14:03 ` [PATCH 17/23] midx: read objects from multi-pack-index Derrick Stolee
2018-06-09 17:56   ` Duy Nguyen
2018-06-21 20:03     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 18/23] midx: use midx in abbreviation calculations Derrick Stolee
2018-06-09 18:01   ` Duy Nguyen
2018-06-22 18:38     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 19/23] midx: use existing midx when writing new one Derrick Stolee
2018-06-07 14:03 ` [PATCH 20/23] midx: use midx in approximate_object_count Derrick Stolee
2018-06-09 18:03   ` Duy Nguyen
2018-06-22 18:39     ` Derrick Stolee
2018-06-07 14:03 ` [PATCH 21/23] midx: prevent duplicate packfile loads Derrick Stolee
2018-06-09 18:05   ` Duy Nguyen
2018-06-07 14:03 ` [PATCH 22/23] midx: use midx to find ref-deltas Derrick Stolee
2018-06-07 14:03 ` [PATCH 23/23] midx: clear midx on repack Derrick Stolee
2018-06-09 18:13   ` Duy Nguyen
2018-06-22 18:44     ` Derrick Stolee
2018-06-07 14:06 ` [PATCH 00/23] Multi-pack-index (MIDX) Derrick Stolee
2018-06-07 14:45 ` Ævar Arnfjörð Bjarmason
2018-06-07 14:54   ` Derrick Stolee
2018-06-25 14:34 ` [PATCH v2 00/24] " Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 01/24] multi-pack-index: add design document Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 02/24] multi-pack-index: add format details Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 03/24] multi-pack-index: add builtin Derrick Stolee
2018-06-25 19:15     ` Junio C Hamano
2018-06-25 14:34   ` [PATCH v2 04/24] multi-pack-index: add 'write' verb Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 05/24] midx: write header information to lockfile Derrick Stolee
2018-06-25 19:19     ` Junio C Hamano
2018-07-05 19:13       ` Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 06/24] multi-pack-index: load into memory Derrick Stolee
2018-06-25 19:38     ` Junio C Hamano
2018-07-05 14:19       ` Derrick Stolee
2018-07-05 18:58         ` Eric Sunshine
2018-07-06 19:20           ` Junio C Hamano
2018-06-25 14:34   ` [PATCH v2 07/24] multi-pack-index: expand test data Derrick Stolee
2018-06-25 19:45     ` Junio C Hamano
2018-06-25 14:34   ` [PATCH v2 08/24] packfile: generalize pack directory list Derrick Stolee
2018-06-25 19:57     ` Junio C Hamano
2018-06-25 14:34   ` [PATCH v2 09/24] multi-pack-index: read packfile list Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 10/24] multi-pack-index: write pack names in chunk Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 11/24] midx: read pack names into array Derrick Stolee
2018-06-25 23:52     ` Eric Sunshine
2018-06-25 14:34   ` [PATCH v2 12/24] midx: sort and deduplicate objects from packfiles Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 13/24] midx: write object ids in a chunk Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 14/24] midx: write object id fanout chunk Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 15/24] midx: write object offsets Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 16/24] config: create core.multiPackIndex setting Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 17/24] midx: prepare midxed_git struct Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 18/24] midx: read objects from multi-pack-index Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 19/24] midx: use midx in abbreviation calculations Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 20/24] midx: use existing midx when writing new one Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 21/24] midx: use midx in approximate_object_count Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 22/24] midx: prevent duplicate packfile loads Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 23/24] packfile: skip loading index if in multi-pack-index Derrick Stolee
2018-06-25 14:34   ` [PATCH v2 24/24] midx: clear midx on repack Derrick Stolee
2018-07-06  0:52   ` [PATCH v3 00/24] Multi-pack-index (MIDX) Derrick Stolee
2018-07-06  0:52     ` [PATCH v3 01/24] multi-pack-index: add design document Derrick Stolee
2018-07-06  0:52     ` [PATCH v3 02/24] multi-pack-index: add format details Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 03/24] multi-pack-index: add builtin Derrick Stolee
2018-07-06  3:54       ` Eric Sunshine
2018-07-06  0:53     ` [PATCH v3 04/24] multi-pack-index: add 'write' verb Derrick Stolee
2018-07-06  4:07       ` Eric Sunshine
2018-07-06  0:53     ` [PATCH v3 05/24] midx: write header information to lockfile Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 06/24] multi-pack-index: load into memory Derrick Stolee
2018-07-06  4:19       ` Eric Sunshine
2018-07-06  5:18         ` Eric Sunshine
2018-07-09 19:08       ` Junio C Hamano
2018-07-12 16:06         ` Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 07/24] multi-pack-index: expand test data Derrick Stolee
2018-07-06  4:36       ` Eric Sunshine
2018-07-06  5:20         ` Eric Sunshine
2018-07-12 14:10         ` Derrick Stolee
2018-07-12 18:02           ` Eric Sunshine
2018-07-12 18:06             ` Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 08/24] packfile: generalize pack directory list Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 09/24] multi-pack-index: read packfile list Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 10/24] multi-pack-index: write pack names in chunk Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 11/24] midx: read pack names into array Derrick Stolee
2018-07-06  4:58       ` Eric Sunshine
2018-07-06  0:53     ` [PATCH v3 12/24] midx: sort and deduplicate objects from packfiles Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 13/24] midx: write object ids in a chunk Derrick Stolee
2018-07-06  5:04       ` Eric Sunshine
2018-07-06  0:53     ` [PATCH v3 14/24] midx: write object id fanout chunk Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 15/24] midx: write object offsets Derrick Stolee
2018-07-06  5:27       ` Eric Sunshine
2018-07-12 16:33         ` Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 16/24] config: create core.multiPackIndex setting Derrick Stolee
2018-07-06  5:39       ` Eric Sunshine
2018-07-12 13:19         ` Derrick Stolee
2018-07-12 16:30           ` Derrick Stolee
2018-07-11  9:48       ` SZEDER Gábor
2018-07-12 13:01         ` Derrick Stolee
2018-07-12 13:31           ` SZEDER Gábor
2018-07-12 15:40             ` Derrick Stolee
2018-07-12 17:29             ` Junio C Hamano
2018-07-06  0:53     ` [PATCH v3 17/24] midx: prepare midxed_git struct Derrick Stolee
2018-07-06  5:41       ` Eric Sunshine
2018-07-06  0:53     ` [PATCH v3 18/24] midx: read objects from multi-pack-index Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 19/24] midx: use midx in abbreviation calculations Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 20/24] midx: use existing midx when writing new one Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 21/24] midx: use midx in approximate_object_count Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 22/24] midx: prevent duplicate packfile loads Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 23/24] packfile: skip loading index if in multi-pack-index Derrick Stolee
2018-07-06  0:53     ` [PATCH v3 24/24] midx: clear midx on repack Derrick Stolee
2018-07-06  5:52       ` Eric Sunshine
2018-07-12 19:39     ` [PATCH v4 00/23] Multi-pack-index (MIDX) Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 01/23] multi-pack-index: add design document Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 02/23] multi-pack-index: add format details Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 03/23] multi-pack-index: add builtin Derrick Stolee
2018-07-20 18:22         ` Junio C Hamano
2018-07-20 22:15           ` brian m. carlson
2018-07-20 22:28             ` Junio C Hamano
2018-07-12 19:39       ` [PATCH v4 04/23] multi-pack-index: add 'write' verb Derrick Stolee
2018-07-12 22:56         ` Eric Sunshine
2018-07-12 19:39       ` [PATCH v4 05/23] midx: write header information to lockfile Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 06/23] multi-pack-index: load into memory Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 07/23] t5319: expand test data Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 08/23] packfile: generalize pack directory list Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 09/23] multi-pack-index: read packfile list Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 10/23] multi-pack-index: write pack names in chunk Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 11/23] midx: read pack names into array Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 12/23] midx: sort and deduplicate objects from packfiles Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 13/23] midx: write object ids in a chunk Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 14/23] midx: write object id fanout chunk Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 15/23] midx: write object offsets Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 16/23] config: create core.multiPackIndex setting Derrick Stolee
2018-07-12 21:05         ` Junio C Hamano
2018-07-13  0:50           ` Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 17/23] midx: read objects from multi-pack-index Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 18/23] midx: use midx in abbreviation calculations Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 19/23] midx: use existing midx when writing new one Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 20/23] midx: use midx in approximate_object_count Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 21/23] midx: prevent duplicate packfile loads Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 22/23] packfile: skip loading index if in multi-pack-index Derrick Stolee
2018-07-12 19:39       ` [PATCH v4 23/23] midx: clear midx on repack Derrick Stolee
2018-07-12 21:11       ` [PATCH v4 00/23] Multi-pack-index (MIDX) Junio C Hamano

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