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Nuno Cruces
2024-05-04 11:44:54 +01:00
parent 5d5c302ff4
commit f0ce3e58eb
2 changed files with 22 additions and 14 deletions
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vfs/README.md
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@@ -24,17 +24,15 @@ OFD locks are fully compatible with POSIX advisory locks.
On BSD Unixes, this module uses
[BSD locks](https://man.freebsd.org/cgi/man.cgi?query=flock&sektion=2).
On BSD, these locks are fully compatible with POSIX advisory locks.
However, concurrency is reduced with BSD locks
On BSD, macOS, and illumos, these locks are fully compatible with POSIX advisory locks;
on Linux and z/OS, they are fully functional, but incompatible with POSIX advisory locks.
However, concurrency is always reduced with BSD locks
(`BEGIN IMMEDIATE` behaves the same as `BEGIN EXCLUSIVE`).
You can opt into BSD locks with the `sqlite3_flock` build tag.
On Windows, this module uses `LockFileEx` and `UnlockFileEx`,
like SQLite.
On Linux and z/OS, BSD locks are fully functional,
but incompatible with POSIX advisory locks (and SQLite).
You can opt into BSD locks with the `sqlite3_flock` build tag.
On all other platforms, file locking is not supported, and you must use
[`nolock=1`](https://sqlite.org/uri.html#urinolock)
(or [`immutable=1`](https://sqlite.org/uri.html#uriimmutable))
@@ -44,7 +42,7 @@ with `nolock=1` you must disable connection pooling by calling
[`db.SetMaxOpenConns(1)`](https://pkg.go.dev/database/sql#DB.SetMaxOpenConns).
You can use [`vfs.SupportsFileLocking`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs#SupportsFileLocking)
to check if your platform supports file locking.
to check if your build supports file locking.
### Write-Ahead Logging
@@ -56,7 +54,7 @@ To allow `mmap` to work, each connection needs to reserve up to 4GB of address s
To limit the amount of address space each connection needs,
use [`WithMemoryLimitPages`](../tests/testcfg/testcfg.go).
On Windows and BSD, [WAL](https://sqlite.org/wal.html) support is
On Windows, and with BSD locks, [WAL](https://sqlite.org/wal.html) support is
[limited](https://sqlite.org/wal.html#noshm).
`EXCLUSIVE` locking mode can be set to create, read, and write WAL databases.\
To use `EXCLUSIVE` locking mode with the
@@ -64,20 +62,29 @@ To use `EXCLUSIVE` locking mode with the
you must disable connection pooling by calling
[`db.SetMaxOpenConns(1)`](https://pkg.go.dev/database/sql#DB.SetMaxOpenConns).
On all other platforms, where file locking is not supported, WAL mode does not work.
On all other platforms, where file locking is not supported, WAL mode is disabled.
You can use [`vfs.SupportsSharedMemory`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs#SupportsSharedMemory)
to check if your platform supports shared memory.
to check if your build supports shared memory.
### Batch-Atomic Write
On 64-bit Linux, this module supports [batch-atomic writes](https://sqlite.org/cgi/src/technote/714)
with the F2FS filesystem.
### Build tags
### Build Tags
The VFS can be customized with a few build tags:
- `sqlite3_flock` forces the use of BSD locks; it can be used on macOS to test the BSD locking implementation.
- `sqlite3_flock` forces the use of BSD locks; it can be used on z/OS for locking support,
and elsewhere to test the BSD locks.
- `sqlite3_nosys` prevents importing [`x/sys`](https://pkg.go.dev/golang.org/x/sys);
disables locking _and_ shared memory on all platforms.
- `sqlite3_noshm` disables shared memory on all platforms.
> [!CAUTION]
> If file locking is incompatible with POSIX advisory locks,
> accessing databases concurrently with this package and other SQLite implementations
> is unsafe, and will eventually corrupt data.
> This will only be the case if you explicitly opt into BSD locks with `sqlite3_flock`.
> The SQLite [`unix-flock`](https://sqlite.org/compile.html#enable_locking_style) VFS
> is always compatible with `sqlite3_flock`.