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Documentation/technical/reftable: change suggested file names 

By using ${min_update}-${max_update} as file name template, we
guarantee that each file has a unique name.
This allows data from open files to be cached across reloads of the
stack.

This is in anticipation of Change I1837f268e 
("file: implement FileReftableDatabase"), which is the first
implementation of reftable on a filesystem.

Change-Id: I7ef0610eb60c494165382d0c372afcf41f074393
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
This commit is contained in:
Han-Wen Nienhuys 2019-08-22 15:47:17 +02:00 committed by Matthias Sohn
parent b4e5ae3b34
commit cf11a03bc2
1 changed files with 15 additions and 11 deletions
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26
Documentation/technical/reftable.md
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@ -775,12 +775,12 @@ directory. This prevents loose references from being stored.
A collection of reftable files are stored in the `$GIT_DIR/reftable/`
directory:
00000001.log
00000001.ref
00000002.ref
00000001-00000001.log
00000002-00000002.ref
00000003-00000003.ref
where reftable files are named by a unique name such as produced by
the function `${update_index}.ref`.
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.
@ -790,9 +790,9 @@ files, one per line, in order, from oldest (base) to newest (most
recent):
$ cat .git/refs
00000001.log
00000001.ref
00000002.ref
00000001-00000001.log
00000002-00000002.ref
00000003-00000003.ref
Readers must read `$GIT_DIR/refs` to determine which files are
relevant right now, and search through the stack in reverse order
@ -819,8 +819,8 @@ new reftable and atomically appending it to the stack:
1. Acquire `refs.lock`.
2. Read `refs` to determine current reftables.
3. Select `update_index` to be most recent file's `max_update_index + 1`.
4. Prepare temp reftable `${update_index}_XXXXXX`, including log entries.
5. Rename `${update_index}_XXXXXX` to `${update_index}.ref`.
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`.
@ -865,12 +865,13 @@ is going to compact B and C, leaving A and D alone.
Ownership of these locks prevents other processes from trying
to compact these files.
3. Release `refs.lock`.
4. Compact `B` and `C` into a temp file `${min_update_index}_XXXXXX`.
4. Compact `B` and `C` into a temp file `${min_update_index}-${max_update_index}_XXXXXX`.
5. Reacquire lock `refs.lock`.
6. Verify that `B` and `C` are still in the stack, in that order. This
should always be the case, assuming that other processes are adhering
to the locking protocol.
7. Rename `${min_update_index}_XXXXXX` to `${min_update_index}_2.ref`.
7. Rename `${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
file from (4).
9. Rename `refs.lock` to `refs`.
@ -879,6 +880,9 @@ is going to compact B and C, leaving A and D alone.
This strategy permits compactions to proceed independently of updates.
Each reftable (compacted or not) is uniquely identified by its name, so open
reftables can be cached by their name.
## Alternatives considered
### bzip packed-refs