Re: [sqlite] PRAGMA journal_size_limit prevents -wal growth or only size outside a transaction?
Howard Kapustein wrote: > /* Try to truncate the WAL file to zero bytes if the checkpoint > ** completed and fsynced (rc==SQLITE_OK) and we are in persistent > ** WAL mode (bPersist) and if the PRAGMA journal_size_limit is a > ** non-negative value (pWal->mxWalSize>=0). Note that we truncate > ** to zero bytes as truncating to the journal_size_limit might > ** leave a corrupt WAL file on disk. */ > walLimitSize(pWal, 0); > > That last comment "Note that we truncate..." is interesting. When > would this corrupt WAL rear its head? A 'normal' checkpoint would adjust the WAL file header to record that the data has been checkpointed and is no longer valid. Regards, Clemens ___ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
Re: [sqlite] PRAGMA journal_size_limit prevents -wal growth or only size outside a transaction?
Looking through the source a bit I see sqlite3_busy_timeout is a no-fail
operation (aside from misuse) so sqlite3_busy_timeout(0);
SQLITE_CHECKPOINT_TRUNCATEsqlite3_busy_timeout(n); does what I need. Thanks
all.
- Howard
P.S. sqlite3_close intentionally doesn't respect journal_size_limit. From
sqlite3WalClose
}else if( pWal->mxWalSize>=0 ){
/* Try to truncate the WAL file to zero bytes if the checkpoint
** completed and fsynced (rc==SQLITE_OK) and we are in persistent
** WAL mode (bPersist) and if the PRAGMA journal_size_limit is a
** non-negative value (pWal->mxWalSize>=0). Note that we truncate
** to zero bytes as truncating to the journal_size_limit might
** leave a corrupt WAL file on disk. */
walLimitSize(pWal, 0);
}
That last comment "Note that we truncate..." is interesting. When would this
corrupt WAL rear its head?
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Re: [sqlite] PRAGMA journal_size_limit prevents -wal growth or only size outside a transaction?
I use journal_mode=WAL and have periods of checkpoint starvation (complicated
reasons) so I'm looking to prune the -wal file but in less blunt way than
SQLITE_CHECKPOINT_TRUNCATE.
Behaviorally I want SQLITE_CHECKPOINT_PASSIVE *and then* if required -wal
content < journal_size_limit, to do the truncation a la
SQLITE_CHECKPOINT_TRUNCATE
SQLITE_CHECKPOINT_PASSIVE gives me the best-effort checkpointing, but in the
best case I don't get -wal shrinkage. SQLITE_CHECKPOINT_TRUNCATE is more of an
aggressive do-it-now-wait-if-necessary which gets me the shrinkage behavior,
but with *required*-effort rather than best-effort. I'd really like both --
best-effort checkpoint AND best-effort truncate.
sqlite3WalClose does exactly what I want (the key bits)
sqlite3OsLock(pWal->pDbFd, SQLITE_LOCK_EXCLUSIVE)
sqlite3WalCheckpoint(pWal, db, SQLITE_CHECKPOINT_PASSIVE,...)
sqlite3OsFileControlHint(pWal->pDbFd, SQLITE_FCNTL_PERSIST_WAL, )
if (bPersist) { if (pWal->mxWalSize>=0) { walLimitSize(pWal, 0) } }
But AFAICT this is only called when PRAGMA journal_mode changes to not WAL or
the pager cache is closed when via sqlite3_close(). I'm a long running process
with connection caching and associated prepared statements so torching the
connection to trigger this isn't optimal.
Can I indirectly get this behavior if I open then immediately close a
connection?
I think so. If there are no other connections to the same database. If
there are any other connections, the checkpoint-on-close won't happen of
course.
I think I can sorta approximate this it if I disable the busy-handler, do
SQLITE_CHECKPOINT_TRUNCATE, ignore Busy/Locked errors and restore the
busy-handler before returning, but that's merely 'sorta' -- ugly on several
levels.
Which part of the sorta is the problem? If you run an
SQLITE_CHECKPOINT_TRUNCATE checkpoint without a busy-handler, or with
the busy-handler rigged to return 0 immediately, then it will:
* attempt a best-effort checkpoint (same as PASSIVE),
* if the entire wal file was checkpointed, check if any readers are
still using it,
* if no readers are still using it, truncate the wal file to zero
bytes in size.
I don't see any way to directly try to best-effort truncate the -wal file e.g.
sqlite3_wal_checkpoint_v2(db, NULL, SQLITE_CHECKPOINT_PASSIVE, NULL, NULL))
sqlite3_file_control(db, NULL, SQLITE_FCNTL_TRUNCATE_WAL, NULL)
Is the idea that this file-control would truncate the wal file to zero
bytes in size iff it could safely do so without blocking on any other
clients?
Dan.
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Re: [sqlite] PRAGMA journal_size_limit prevents -wal growth or only size outside a transaction?
The journal_size_limit in WAL mode is for when the WAL file resets (everything
checkpointed successfully). It doesn't limit transaction size in any way. It's
simply "when everything has checkpointed: cut the file back to at most this
size" rather than the normal mode of: "wal file size will never shrink, just be
recycled", (similar to how the database file size doesn't shrink when you
delete records).
Can't help you on the C code part though, sorry.
-Original Message-
From: sqlite-users [mailto:sqlite-users-boun...@mailinglists.sqlite.org] On
Behalf Of Howard Kapustein
Sent: Tuesday, September 05, 2017 1:07 PM
To: sqlite-users@mailinglists.sqlite.org
Subject: [sqlite] PRAGMA journal_size_limit prevents -wal growth or only size
outside a transaction?
The docs are a little unclear =>
https://sqlite.org/pragma.html#pragma_journal_size_limit
I need to disable autocheckpoint@close (for other reasons) so I'm looking for
ways to fence the -wal file. If I want to bound a database's -wal file to <=1MB
when I'm not in a transaction is it just PRAGMA journal_size_limit=1048576;?
Does this affect -wal growth *during* a transaction, i.e. if I BEGIN; ...INSERT
lots...; COMMIT will I successfully reach COMMIT? The journal_size_limit
doesn't affect *growth* of the -wal, right? Just 'at rest' size (i.e. outside
any transaction)?
As for why... I'm good with -wal growth while I do work but in some cases I've
got too much work coming and going w/o any idle period for autocheckpoint@close
to kick in. In a few cases I see huge (700MB+) -wal files when I've got no
transaction in play. I've set SQLITE_FCNTL_PERSIST_WAL=1 and want to explicitly
checkpoint similar to how sqlite does on close i.e. Passive + truncate
SQLITE_PRIVATE int sqlite3WalClose(...){
...
rc = sqlite3WalCheckpoint(pWal, db,
SQLITE_CHECKPOINT_PASSIVE, 0, 0, sync_flags, nBuf, zBuf, 0, 0
);
...
}else if( pWal->mxWalSize>=0 ){
/* Try to truncate the WAL file to zero bytes if the checkpoint
** completed and fsynced (rc==SQLITE_OK) and we are in persistent
** WAL mode (bPersist) and if the PRAGMA journal_size_limit is a
** non-negative value (pWal->mxWalSize>=0). Note that we truncate
** to zero bytes as truncating to the journal_size_limit might
** leave a corrupt WAL file on disk. */
walLimitSize(pWal, 0);
}
}
}
Is this the functional equivalent?
int limit = ...get current value via sqlite3_exec(db, "PRAGMA
journal_size_limit;")...
sqlite3_exec(db, "PRAGMA journal_size_limit=0;")
sqlite3_wal_checkpoint_v2(db, NULL, SQLITE_CHECKPOINT_PASSIVE, , );
sqlite3_exec(db, "PRAGMA journal_size_limit=" + limit + ";")
Thanks,
* Howard
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