pageserver: reduce import memory utilization (#12086)

## Problem

Imports can end up allocating too much.

## Summary of Changes

Nerf them a bunch and add some logs.

pageserver/src/tenant/timeline/import_pgdata.rs

@@ -106,6 +106,8 @@ pub async fn doit(
                 );
             }
 
+            tracing::info!("Import plan executed. Flushing remote changes and notifying storcon");
+
             timeline
                 .remote_client
                 .schedule_index_upload_for_file_changes()?;

pageserver/src/tenant/timeline/import_pgdata/flow.rs

@@ -130,7 +130,15 @@ async fn run_v1(
 
     pausable_failpoint!("import-timeline-pre-execute-pausable");
 
+    let jobs_count = import_progress.as_ref().map(|p| p.jobs);
     let start_from_job_idx = import_progress.map(|progress| progress.completed);
+
+    tracing::info!(
+        start_from_job_idx=?start_from_job_idx,
+        jobs=?jobs_count,
+        "Executing import plan"
+    );
+
     plan.execute(timeline, start_from_job_idx, plan_hash, &import_config, ctx)
         .await
 }
@@ -484,6 +492,8 @@ impl Plan {
                                     anyhow::anyhow!("Shut down while putting timeline import status")
                                 })?;
                             }
+
+                            tracing::info!(last_completed_job_idx, jobs=%jobs_in_plan, "Checkpointing import status");
                         },
                         Some(Err(_)) => {
                             anyhow::bail!(
@@ -760,7 +770,7 @@ impl ImportTask for ImportRelBlocksTask {
         layer_writer: &mut ImageLayerWriter,
         ctx: &RequestContext,
     ) -> anyhow::Result<usize> {
-        const MAX_BYTE_RANGE_SIZE: usize = 128 * 1024 * 1024;
+        const MAX_BYTE_RANGE_SIZE: usize = 4 * 1024 * 1024;
 
         debug!("Importing relation file");