querylog: checkpoint every 8192 wal pages instead of 1000

src/storage/querylog_schema.zig

@@ -77,6 +77,31 @@ pub const fingerprint: i32 = blk: {
 
 const set_user_version = std.fmt.comptimePrint("PRAGMA user_version = {d};", .{fingerprint});
 
+/// The WAL checkpoint threshold for every read-write `querylog.db` connection,
+/// in pages (32 MiB at the 4096-byte page size). SQLite's 1000-page default
+/// trips every ~40 min under this workload and rewrites the same hot index and
+/// interior pages into the main database each time; 8192 stretches that to ~5 h
+/// and cuts those in-place rewrites ~8x, which is SD-card write wear this
+/// household appliance does not need to spend.
+///
+/// The durability consequence, stated precisely:
+///
+/// - Commit never fsyncs at `synchronous = NORMAL`; the checkpoint's fsync is
+///   the only guaranteed durability boundary. This moves that boundary from
+///   ~40 min to ~5 h of querylog data (query rows plus upstream-history
+///   minutes) under power loss or kernel panic. Typical loss stays far smaller
+///   because of kernel writeback, but that is not a guarantee.
+/// - Process crash or clean stop loses nothing committed, at any threshold.
+///   Consistency is never at risk: recovery replays the longest valid WAL
+///   prefix atomically.
+/// - 32 MiB is an expectation, not a cap: a pinned reader snapshot stops a
+///   passive checkpoint partway and the WAL overshoots until that reader
+///   finishes. The daily retention `wal_checkpoint(TRUNCATE)` is the backstop
+///   that shrinks the file.
+///
+/// `config.db` keeps the SQLite default: it holds configuration, not a log.
+pub const wal_autocheckpoint_pages: i32 = 8192;
+
 /// Long enough for any path this program will be handed, plus the aside suffix.
 /// A longer path is `error.NameTooLong`, which is what the filesystem calls
 /// would have returned anyway.
@@ -126,7 +151,7 @@ pub fn open(io: std.Io, dir: std.Io.Dir, path: [:0]const u8) Error!OpenResult {
             break :probe recreatable(e) orelse return e;
         const opened = &handle.?;
 
-        db.applyPragmas(opened, .{}) catch |e|
+        db.applyPragmas(opened, .{ .wal_autocheckpoint_pages = wal_autocheckpoint_pages }) catch |e|
             break :probe recreatable(e) orelse return e;
 
         const healthy = quickCheck(opened) catch |e|
@@ -250,7 +275,7 @@ fn deleteSidecars(io: std.Io, dir: std.Io.Dir, path: []const u8) Error!void {
 fn createFresh(path: [:0]const u8) db.Error!db.Db {
     var database = try db.Db.open(path, .{ .mode = .read_write_create });
     errdefer database.close();
-    try db.applyPragmas(&database, .{});
+    try db.applyPragmas(&database, .{ .wal_autocheckpoint_pages = wal_autocheckpoint_pages });
 
     var tx = try db.Tx.begin(&database);
     errdefer tx.rollback();