I would like for a new repo version to enable appends, but to do so
safely would need a v11 followed by a 1 year delay followed by a v12
that does it. Since a similar v9 and v10 transition is currently
happening, and is less than 6 months along in most repos, it does not
feel wise to stack up another year-long transition behind that. What if
I need to hurry up a new repo version for some other change?
Added todo so I remember to make this change at some time when a v11
and probably v12 repo version do make sense.
Sponsored-by: Dartmouth College's DANDI project
An append that is interrupted and writes part of a line is now dealt
with by subsequent reads and appends. This also handles a read that
happens at the same time as an append to the file.
Old versions of git-annex will still see a partially written line,
and could get confused. Since appends are currently done for url logs
and location logs, the confusion is limited to a substring of the actual
url or UUID of the remote being read. This will not affect writes, since
the journal file is locked when reading in preparation for writing.
However, the bad data can be output by git-annex and used by other
things, or could cause surprising behavior by git-annex. Including eg,
downloading the content of the wrong url.
So, something needs to be done to prevent old versions of git-annex from
running in a repository where this appending is being done..
Sponsored-by: Dartmouth College's DANDI project
This reverts commit 2e6e9876e3.
This is gonna be needed after all.. The append will only be atomic if
the journal is locked, because the file being appended will have to be
moved out of the way to avoid an old version of git-annex seeing an
incomplete write to it. When git-annex finds that the file is not in the
journal, and checks the append location, locking will be needed to avoid
a race causing it to miss it in the append location too due to it being
moved back to the journal.
Added annex.alwayscompact setting which can be unset to speed up writes to
the git-annex branch in some cases.
Sponsored-by: Dartmouth College's DANDI project
This reverts commit 47358a6f95.
This added overhead, and will not be needed, because appends are going
to have to be made atomic for other reasons than avoiding incomplete
reads of data being appended.
In particular, when git-annex is interrupted in the middle of an append,
it must not leave the file with a partially written line. So appending
has to somehow be made fully atomic.
Currently this is not an improvement, but it allows for optimising
appendJournalFile later. With an optimised appendJournalFile, this will
greatly speed up access patterns like git-annex addurl of a lot of urls
to the same key, where the log file can grow rather large. Appending
rather than re-writing the journal file for each line can save a lot of
disk writes.
It still has to read the current journal or branch file, to check
if it can append to it, and so when the journal file does not exist yet,
it can write the old content from the branch to it. Probably the re-reads
are better cached by the filesystem than repeated writes. (If the
re-reads turn out to keep performance bad, they could be eliminated, at
the cost of not being able to compact the log when replacing old
information in it. That could be enabled by a switch.)
While the immediate need is to affect addurl writes, it was implemented
at the level of presence logs, so will also perhaps speed up location logs.
The only added overhead is the call to isNewInfo, which only needs to
compare ByteStrings. Helping to balance that out, it avoids compactLog
when it's able to append.
Sponsored-by: Dartmouth College's DANDI project
It started complaining about custom setup needing too old a version of
cabal, a very confusing error message.
1.12 is the version of Cabal on the i386ancient builder.
Sponsored-by: Jack Hill on Patreon
This is not currently necessary; journal files are updated atomically.
However, for faster appends to large journal files, locking on read will
be needed, because appends are not atomic.
Sponsored-by: Dartmouth College's DANDI project
Fix a reversion that prevented --batch commands (and the assistant)
from noticing data written to the journal by other commands.
I have not identified which commit broke this for sure,
but probably it was aeca7c2207
--batch commands that wrote to the journal avoided the problem since
journalIgnorable sets unset on write. It's a little bit surprising that
nobody noticed that query --batch commands did not see data written by
other commands.
Sponsored-by: Dartmouth College's DANDI project
