review: add coment for max vec read bytes constant

libs/pageserver_api/src/config.rs

@@ -296,6 +296,12 @@ pub mod defaults {
 
     pub const DEFAULT_INGEST_BATCH_SIZE: u64 = 100;
 
+    // Soft limit for the maximum size of a vectored read.
+    // This is determined by the largest NeonWalRecord that can exist (minus dbdir and reldir keys
+    // which are unbounded). As of this writing, that is a `NeonWalRecord::ClogSetCommitted` record,
+    // with 32k xids. That's the max number of XIDS on a single CLOG page. The size of such a record
+    // is `sizeof(Transactionid) * 32768 + (some fixed overhead from 'timestamp`, the Vec length and whatever extra serde serialization adds)`.
+    // That is, slightly above 128 kB.
     pub const DEFAULT_MAX_VECTORED_READ_BYTES: usize = 130 * 1024; // 130 KiB
 
     pub const DEFAULT_IMAGE_COMPRESSION: ImageCompressionAlgorithm =