Split index backfill table iterator page size knob (#42501)

INDEX_BACKFILL_CHUNK_SIZE controls write chunks, INDEX_BACKFILL_READ_SIZE controls TableIterator pages.

GitOrigin-RevId: a43caa579e21a5f339191a725ac9bb47b930a08a

Author: goffrie

crates/common/src/knobs.rs

@@ -425,6 +425,10 @@ pub static ENABLE_INDEX_BACKFILL: LazyLock<bool> =
 pub static INDEX_BACKFILL_CHUNK_RATE: LazyLock<NonZeroU32> =
     LazyLock::new(|| env_config("INDEX_BACKFILL_CHUNK_RATE", NonZeroU32::new(16).unwrap()));
 
+/// The page size to use when reading the table for an index backfill.
+pub static INDEX_BACKFILL_READ_SIZE: LazyLock<usize> =
+    LazyLock::new(|| env_config("INDEX_BACKFILL_READ_SIZE", 500));
+
 /// How many index entries to write within a single database transaction.
 /// Value is a tradeoff between grouping work, vs tying up resources on the
 /// database, vs holding all entries in memory.

crates/database/src/database_index_workers/index_writer.rs

@@ -16,6 +16,7 @@ use common::{
         INDEX_BACKFILL_CHUNK_RATE,
         INDEX_BACKFILL_CHUNK_SIZE,
         INDEX_BACKFILL_PROGRESS_INTERVAL,
+        INDEX_BACKFILL_READ_SIZE,
         INDEX_BACKFILL_WORKERS,
     },
     persistence::{
@@ -266,7 +267,7 @@ impl<RT: Runtime> IndexWriter<RT> {
             snapshot_ts,
             self.reader.clone(),
             self.retention_validator.clone(),
-            INDEX_BACKFILL_CHUNK_SIZE.get() as usize,
+            *INDEX_BACKFILL_READ_SIZE,
         );
 
         let (index_update_tx, index_update_rx) = mpsc::channel(32);