Introduce a maximum size for values and warn for them

pageserver/src/repository.rs

@@ -33,6 +33,9 @@ impl Value {
     }
 }
 
+/// The maximum size of a value supported by the pageserver
+pub const MAX_VALUE_SIZE: usize = 10_000_000;
+
 #[cfg(test)]
 mod test {
     use super::*;

pageserver/src/tenant/storage_layer/inmemory_layer.rs

@@ -6,7 +6,7 @@
 //!
 use crate::config::PageServerConf;
 use crate::context::{PageContentKind, RequestContext, RequestContextBuilder};
-use crate::repository::{Key, Value};
+use crate::repository::{Key, Value, MAX_VALUE_SIZE};
 use crate::tenant::block_io::BlockReader;
 use crate::tenant::ephemeral_file::EphemeralFile;
 use crate::tenant::storage_layer::{ValueReconstructResult, ValueReconstructState};
@@ -284,6 +284,15 @@ impl InMemoryLayer {
     ) -> Result<()> {
         trace!("put_value key {} at {}/{}", key, self.timeline_id, lsn);
 
+        if let Value::Image(buf) = val {
+            if buf.len() > MAX_VALUE_SIZE {
+                tracing::warn!(
+                    "Can't put value of size {} above limit {MAX_VALUE_SIZE} for key {key}",
+                    buf.len()
+                );
+            }
+        }
+
         let off = {
             // Avoid doing allocations for "small" values.
             // In the regression test suite, the limit of 256 avoided allocations in 95% of cases: