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Merge "Documentation/technical/reftable: improve repo layout"

stable-5.8
Jonathan Nieder 5 years ago committed by Gerrit Code Review @ Eclipse.org
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5c6e6f608d
  1. 53
      Documentation/technical/reftable.md

53
Documentation/technical/reftable.md

@ -773,9 +773,6 @@ A repository must set its `$GIT_DIR/config` to configure reftable:
### Layout
The `$GIT_DIR/refs` path is a file when reftable is configured, not a
directory. This prevents loose references from being stored.
A collection of reftable files are stored in the `$GIT_DIR/reftable/`
directory:
@ -789,28 +786,38 @@ the function `${min_update_index}-${max_update_index}.ref`.
Log-only files use the `.log` extension, while ref-only and mixed ref
and log files use `.ref`. extension.
The stack ordering file is `$GIT_DIR/refs` and lists the current
files, one per line, in order, from oldest (base) to newest (most
recent):
$ cat .git/refs
The stack ordering file is `$GIT_DIR/reftable/tables.list` and lists the current
files, one per line, in order, from oldest (base) to newest (most recent):
$ cat .git/reftable/tables.list
00000001-00000001.log
00000002-00000002.ref
00000003-00000003.ref
Readers must read `$GIT_DIR/refs` to determine which files are
relevant right now, and search through the stack in reverse order
(last reftable is examined first).
Readers must read `$GIT_DIR/reftable/tables.list` to determine which files are
relevant right now, and search through the stack in reverse order (last reftable
is examined first).
Reftable files not listed in `refs` may be new (and about to be added
Reftable files not listed in `tables.list` may be new (and about to be added
to the stack by the active writer), or ancient and ready to be pruned.
### Backward compatibility
Older clients should continue to recognize the directory as a git repository so
they don't look for an enclosing repository in parent directories. To this end,
a reftable-enabled repository must contain the following dummy files
* `.git/HEAD`, a regular file containing `ref: refs/heads/.invalid`.
* `.git/refs/`, a directory
* `.git/refs/heads`, a regular file
### Readers
Readers can obtain a consistent snapshot of the reference space by
following:
1. Open and read the `refs` file.
1. Open and read the `tables.list` file.
2. Open each of the reftable files that it mentions.
3. If any of the files is missing, goto 1.
4. Read from the now-open files as long as necessary.
@ -820,13 +827,13 @@ following:
Although reftables are immutable, mutations are supported by writing a
new reftable and atomically appending it to the stack:
1. Acquire `refs.lock`.
2. Read `refs` to determine current reftables.
1. Acquire `tables.list.lock`.
2. Read `tables.list` to determine current reftables.
3. Select `update_index` to be most recent file's `max_update_index + 1`.
4. Prepare temp reftable `tmp_XXXXXX`, including log entries.
5. Rename `tmp_XXXXXX` to `${update_index}-${update_index}.ref`.
6. Copy `refs` to `refs.lock`, appending file from (5).
7. Rename `refs.lock` to `refs`.
6. Copy `tables.list` to `tables.list.lock`, appending file from (5).
7. Rename `tables.list.lock` to `tables.list`.
During step 4 the new file's `min_update_index` and `max_update_index`
are both set to the `update_index` selected by step 3. All log
@ -834,9 +841,9 @@ records for the transaction use the same `update_index` in their keys.
This enables later correlation of which references were updated by the
same transaction.
Because a single `refs.lock` file is used to manage locking, the
Because a single `tables.list.lock` file is used to manage locking, the
repository is single-threaded for writers. Writers may have to
busy-spin (with backoff) around creating `refs.lock`, for up to an
busy-spin (with backoff) around creating `tables.list.lock`, for up to an
acceptable wait period, aborting if the repository is too busy to
mutate. Application servers wrapped around repositories (e.g. Gerrit
Code Review) can layer their own lock/wait queue to improve fairness
@ -864,21 +871,21 @@ For sake of illustration, assume the stack currently consists of
reftable files (from oldest to newest): A, B, C, and D. The compactor
is going to compact B and C, leaving A and D alone.
1. Obtain lock `refs.lock` and read the `refs` file.
1. Obtain lock `tables.list.lock` and read the `tables.list` file.
2. Obtain locks `B.lock` and `C.lock`.
Ownership of these locks prevents other processes from trying
to compact these files.
3. Release `refs.lock`.
3. Release `tables.list.lock`.
4. Compact `B` and `C` into a temp file `${min_update_index}-${max_update_index}_XXXXXX`.
5. Reacquire lock `refs.lock`.
5. Reacquire lock `tables.list.lock`.
6. Verify that `B` and `C` are still in the stack, in that order. This
should always be the case, assuming that other processes are adhering
to the locking protocol.
7. Rename `${min_update_index}-${max_update_index}_XXXXXX` to
`${min_update_index}-${max_update_index}.ref`.
8. Write the new stack to `refs.lock`, replacing `B` and `C` with the
8. Write the new stack to `tables.list.lock`, replacing `B` and `C` with the
file from (4).
9. Rename `refs.lock` to `refs`.
9. Rename `tables.list.lock` to `tables.list`.
10. Delete `B` and `C`, perhaps after a short sleep to avoid forcing
readers to backtrack.

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