pageserver: tweak oversized key read path warning (#9221)

## Problem

`Oversized vectored read [...]` logs are spewing in prod because we have
a few keys that
are unexpectedly large:
* reldir/relblock - these are unbounded, so it's known technical debt
* slru block - they can be a bit bigger than 128KiB due to storage
format overhead

## Summary of changes

* Bump threshold to 130KiB
* Don't warn on oversized reldir and dbdir keys 

Closes https://github.com/neondatabase/neon/issues/8967

libs/pageserver_api/src/config.rs

@@ -296,7 +296,14 @@ pub mod defaults {
 
     pub const DEFAULT_INGEST_BATCH_SIZE: u64 = 100;
 
-    pub const DEFAULT_MAX_VECTORED_READ_BYTES: usize = 128 * 1024; // 128 KiB
+    /// 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 bounded by the blob io limits only). 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 =
         ImageCompressionAlgorithm::Zstd { level: Some(1) };

libs/pageserver_api/src/key.rs

@@ -748,6 +748,16 @@ impl Key {
         self.field1 == 0x00 && self.field4 != 0 && self.field6 != 0xffffffff
     }
 
+    #[inline(always)]
+    pub fn is_rel_dir_key(&self) -> bool {
+        self.field1 == 0x00
+            && self.field2 != 0
+            && self.field3 != 0
+            && self.field4 == 0
+            && self.field5 == 0
+            && self.field6 == 1
+    }
+
     /// Guaranteed to return `Ok()` if [`Self::is_rel_block_key`] returns `true` for `key`.
     #[inline(always)]
     pub fn to_rel_block(self) -> anyhow::Result<(RelTag, BlockNumber)> {

pageserver/src/tenant/storage_layer/delta_layer.rs

@@ -53,6 +53,7 @@ use camino::{Utf8Path, Utf8PathBuf};
 use futures::StreamExt;
 use itertools::Itertools;
 use pageserver_api::config::MaxVectoredReadBytes;
+use pageserver_api::key::DBDIR_KEY;
 use pageserver_api::keyspace::KeySpace;
 use pageserver_api::models::ImageCompressionAlgorithm;
 use pageserver_api::shard::TenantShardId;
@@ -963,14 +964,25 @@ impl DeltaLayerInner {
                 .blobs_at
                 .as_slice()
                 .iter()
-                .map(|(_, blob_meta)| format!("{}@{}", blob_meta.key, blob_meta.lsn))
+                .filter_map(|(_, blob_meta)| {
+                    if blob_meta.key.is_rel_dir_key() || blob_meta.key == DBDIR_KEY {
+                        // The size of values for these keys is unbounded and can
+                        // grow very large in pathological cases.
+                        None
+                    } else {
+                        Some(format!("{}@{}", blob_meta.key, blob_meta.lsn))
+                    }
+                })
                 .join(", ");
-            tracing::warn!(
-                "Oversized vectored read ({} > {}) for keys {}",
-                largest_read_size,
-                read_size_soft_max,
-                offenders
-            );
+
+            if !offenders.is_empty() {
+                tracing::warn!(
+                    "Oversized vectored read ({} > {}) for keys {}",
+                    largest_read_size,
+                    read_size_soft_max,
+                    offenders
+                );
+            }
         }
 
         largest_read_size

pageserver/src/tenant/storage_layer/image_layer.rs

@@ -49,6 +49,7 @@ use camino::{Utf8Path, Utf8PathBuf};
 use hex;
 use itertools::Itertools;
 use pageserver_api::config::MaxVectoredReadBytes;
+use pageserver_api::key::DBDIR_KEY;
 use pageserver_api::keyspace::KeySpace;
 use pageserver_api::shard::{ShardIdentity, TenantShardId};
 use rand::{distributions::Alphanumeric, Rng};
@@ -587,14 +588,25 @@ impl ImageLayerInner {
                     .blobs_at
                     .as_slice()
                     .iter()
-                    .map(|(_, blob_meta)| format!("{}@{}", blob_meta.key, blob_meta.lsn))
+                    .filter_map(|(_, blob_meta)| {
+                        if blob_meta.key.is_rel_dir_key() || blob_meta.key == DBDIR_KEY {
+                            // The size of values for these keys is unbounded and can
+                            // grow very large in pathological cases.
+                            None
+                        } else {
+                            Some(format!("{}@{}", blob_meta.key, blob_meta.lsn))
+                        }
+                    })
                     .join(", ");
-                tracing::warn!(
-                    "Oversized vectored read ({} > {}) for keys {}",
-                    buf_size,
-                    max_vectored_read_bytes,
-                    offenders
-                );
+
+                if !offenders.is_empty() {
+                    tracing::warn!(
+                        "Oversized vectored read ({} > {}) for keys {}",
+                        buf_size,
+                        max_vectored_read_bytes,
+                        offenders
+                    );
+                }
             }
 
             let buf = BytesMut::with_capacity(buf_size);