feat(pageserver): filter keys with gc-compaction (#9004)

Part of https://github.com/neondatabase/neon/issues/8002

Close https://github.com/neondatabase/neon/issues/8920

Legacy compaction (as well as gc-compaction) rely on the GC process to
remove unused layer files, but this relies on many factors (i.e., key
partition) to ensure data in a dropped table can be eventually removed.

In gc-compaction, we consider the keyspace information when doing the
compaction process. If a key is not in the keyspace, we will skip that
key and not include it in the final output.

However, this is not easy to implement because gc-compaction considers
branch points (i.e., retain_lsns) and the retained keyspaces could
change across different LSNs. Therefore, for now, we only remove aux v1
keys in the compaction process.

## Summary of changes

* Add `FilterIterator` to filter out keys.
* Integrate `FilterIterator` with gc-compaction.
* Add `collect_gc_compaction_keyspace` for a spec of keyspaces that can
be retained during the gc-compaction process.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

pageserver/src/pgdatadir_mapping.rs

@@ -840,6 +840,36 @@ impl Timeline {
         Ok(total_size * BLCKSZ as u64)
     }
 
+    /// Get a KeySpace that covers all the Keys that are in use at AND below the given LSN. This is only used
+    /// for gc-compaction.
+    ///
+    /// gc-compaction cannot use the same `collect_keyspace` function as the legacy compaction because it
+    /// processes data at multiple LSNs and needs to be aware of the fact that some key ranges might need to
+    /// be kept only for a specific range of LSN.
+    ///
+    /// Consider the case that the user created branches at LSN 10 and 20, where the user created a table A at
+    /// LSN 10 and dropped that table at LSN 20. `collect_keyspace` at LSN 10 will return the key range
+    /// corresponding to that table, while LSN 20 won't. The keyspace info at a single LSN is not enough to
+    /// determine which keys to retain/drop for gc-compaction.
+    ///
+    /// For now, it only drops AUX-v1 keys. But in the future, the function will be extended to return the keyspace
+    /// to be retained for each of the branch LSN.
+    ///
+    /// The return value is (dense keyspace, sparse keyspace).
+    pub(crate) async fn collect_gc_compaction_keyspace(
+        &self,
+    ) -> Result<(KeySpace, SparseKeySpace), CollectKeySpaceError> {
+        let metadata_key_begin = Key::metadata_key_range().start;
+        let aux_v1_key = AUX_FILES_KEY;
+        let dense_keyspace = KeySpace {
+            ranges: vec![Key::MIN..aux_v1_key, aux_v1_key.next()..metadata_key_begin],
+        };
+        Ok((
+            dense_keyspace,
+            SparseKeySpace(KeySpace::single(Key::metadata_key_range())),
+        ))
+    }
+
     ///
     /// Get a KeySpace that covers all the Keys that are in use at the given LSN.
     /// Anything that's not listed maybe removed from the underlying storage (from

pageserver/src/tenant/storage_layer.rs

@@ -1,13 +1,13 @@
 //! Common traits and structs for layers
 
 pub mod delta_layer;
+pub mod filter_iterator;
 pub mod image_layer;
 pub mod inmemory_layer;
 pub(crate) mod layer;
 mod layer_desc;
 mod layer_name;
 pub mod merge_iterator;
-
 pub mod split_writer;
 
 use crate::context::{AccessStatsBehavior, RequestContext};

pageserver/src/tenant/storage_layer/filter_iterator.rs

@@ -0,0 +1,205 @@
+use std::ops::Range;
+
+use anyhow::bail;
+use pageserver_api::{
+    key::Key,
+    keyspace::{KeySpace, SparseKeySpace},
+};
+use utils::lsn::Lsn;
+
+use crate::repository::Value;
+
+use super::merge_iterator::MergeIterator;
+
+/// A filter iterator over merge iterators (and can be easily extended to other types of iterators).
+///
+/// The iterator will skip any keys not included in the keyspace filter. In other words, the keyspace filter contains the keys
+/// to be retained.
+pub struct FilterIterator<'a> {
+    inner: MergeIterator<'a>,
+    retain_key_filters: Vec<Range<Key>>,
+    current_filter_idx: usize,
+}
+
+impl<'a> FilterIterator<'a> {
+    pub fn create(
+        inner: MergeIterator<'a>,
+        dense_keyspace: KeySpace,
+        sparse_keyspace: SparseKeySpace,
+    ) -> anyhow::Result<Self> {
+        let mut retain_key_filters = Vec::new();
+        retain_key_filters.extend(dense_keyspace.ranges);
+        retain_key_filters.extend(sparse_keyspace.0.ranges);
+        retain_key_filters.sort_by(|a, b| a.start.cmp(&b.start));
+        // Verify key filters are non-overlapping and sorted
+        for window in retain_key_filters.windows(2) {
+            if window[0].end > window[1].start {
+                bail!(
+                    "Key filters are overlapping: {:?} and {:?}",
+                    window[0],
+                    window[1]
+                );
+            }
+        }
+        Ok(Self {
+            inner,
+            retain_key_filters,
+            current_filter_idx: 0,
+        })
+    }
+
+    pub async fn next(&mut self) -> anyhow::Result<Option<(Key, Lsn, Value)>> {
+        while let Some(item) = self.inner.next().await? {
+            while self.current_filter_idx < self.retain_key_filters.len()
+                && item.0 >= self.retain_key_filters[self.current_filter_idx].end
+            {
+                // [filter region]    [filter region]     [filter region]
+                //                                     ^ item
+                //                    ^ current filter
+                self.current_filter_idx += 1;
+                // [filter region]    [filter region]     [filter region]
+                //                                     ^ item
+                //                                        ^ current filter
+            }
+            if self.current_filter_idx >= self.retain_key_filters.len() {
+                // We already exhausted all filters, so we should return now
+                // [filter region] [filter region] [filter region]
+                //                                                    ^ item
+                //                                                 ^ current filter (nothing)
+                return Ok(None);
+            }
+            if self.retain_key_filters[self.current_filter_idx].contains(&item.0) {
+                // [filter region]    [filter region]     [filter region]
+                //                                              ^ item
+                //                                        ^ current filter
+                return Ok(Some(item));
+            }
+            // If the key is not contained in the key retaining filters, continue to the next item.
+            // [filter region]    [filter region]     [filter region]
+            //                                     ^ item
+            //                                        ^ current filter
+        }
+        Ok(None)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use itertools::Itertools;
+    use pageserver_api::key::Key;
+    use utils::lsn::Lsn;
+
+    use crate::{
+        tenant::{
+            harness::{TenantHarness, TIMELINE_ID},
+            storage_layer::delta_layer::test::produce_delta_layer,
+        },
+        DEFAULT_PG_VERSION,
+    };
+
+    async fn assert_filter_iter_equal(
+        filter_iter: &mut FilterIterator<'_>,
+        expect: &[(Key, Lsn, Value)],
+    ) {
+        let mut expect_iter = expect.iter();
+        loop {
+            let o1 = filter_iter.next().await.unwrap();
+            let o2 = expect_iter.next();
+            assert_eq!(o1.is_some(), o2.is_some());
+            if o1.is_none() && o2.is_none() {
+                break;
+            }
+            let (k1, l1, v1) = o1.unwrap();
+            let (k2, l2, v2) = o2.unwrap();
+            assert_eq!(&k1, k2);
+            assert_eq!(l1, *l2);
+            assert_eq!(&v1, v2);
+        }
+    }
+
+    #[tokio::test]
+    async fn filter_keyspace_iterator() {
+        use crate::repository::Value;
+        use bytes::Bytes;
+
+        let harness = TenantHarness::create("filter_iterator_filter_keyspace_iterator")
+            .await
+            .unwrap();
+        let (tenant, ctx) = harness.load().await;
+
+        let tline = tenant
+            .create_test_timeline(TIMELINE_ID, Lsn(0x10), DEFAULT_PG_VERSION, &ctx)
+            .await
+            .unwrap();
+
+        fn get_key(id: u32) -> Key {
+            let mut key = Key::from_hex("000000000033333333444444445500000000").unwrap();
+            key.field6 = id;
+            key
+        }
+        const N: usize = 100;
+        let test_deltas1 = (0..N)
+            .map(|idx| {
+                (
+                    get_key(idx as u32),
+                    Lsn(0x20 * ((idx as u64) % 10 + 1)),
+                    Value::Image(Bytes::from(format!("img{idx:05}"))),
+                )
+            })
+            .collect_vec();
+        let resident_layer_1 = produce_delta_layer(&tenant, &tline, test_deltas1.clone(), &ctx)
+            .await
+            .unwrap();
+
+        let merge_iter = MergeIterator::create(
+            &[resident_layer_1.get_as_delta(&ctx).await.unwrap()],
+            &[],
+            &ctx,
+        );
+
+        let mut filter_iter = FilterIterator::create(
+            merge_iter,
+            KeySpace {
+                ranges: vec![
+                    get_key(5)..get_key(10),
+                    get_key(20)..get_key(30),
+                    get_key(90)..get_key(110),
+                    get_key(1000)..get_key(2000),
+                ],
+            },
+            SparseKeySpace(KeySpace::default()),
+        )
+        .unwrap();
+        let mut result = Vec::new();
+        result.extend(test_deltas1[5..10].iter().cloned());
+        result.extend(test_deltas1[20..30].iter().cloned());
+        result.extend(test_deltas1[90..100].iter().cloned());
+        assert_filter_iter_equal(&mut filter_iter, &result).await;
+
+        let merge_iter = MergeIterator::create(
+            &[resident_layer_1.get_as_delta(&ctx).await.unwrap()],
+            &[],
+            &ctx,
+        );
+
+        let mut filter_iter = FilterIterator::create(
+            merge_iter,
+            KeySpace {
+                ranges: vec![
+                    get_key(0)..get_key(10),
+                    get_key(20)..get_key(30),
+                    get_key(90)..get_key(95),
+                ],
+            },
+            SparseKeySpace(KeySpace::default()),
+        )
+        .unwrap();
+        let mut result = Vec::new();
+        result.extend(test_deltas1[0..10].iter().cloned());
+        result.extend(test_deltas1[20..30].iter().cloned());
+        result.extend(test_deltas1[90..95].iter().cloned());
+        assert_filter_iter_equal(&mut filter_iter, &result).await;
+    }
+}

pageserver/src/tenant/timeline/compaction.rs

@@ -31,6 +31,7 @@ use crate::context::{AccessStatsBehavior, RequestContext, RequestContextBuilder}
 use crate::page_cache;
 use crate::tenant::checks::check_valid_layermap;
 use crate::tenant::remote_timeline_client::WaitCompletionError;
+use crate::tenant::storage_layer::filter_iterator::FilterIterator;
 use crate::tenant::storage_layer::merge_iterator::MergeIterator;
 use crate::tenant::storage_layer::split_writer::{
     SplitDeltaLayerWriter, SplitImageLayerWriter, SplitWriterResult,
@@ -1772,6 +1773,7 @@ impl Timeline {
             gc_cutoff,
             lowest_retain_lsn
         );
+
         // Step 1: (In the future) construct a k-merge iterator over all layers. For now, simply collect all keys + LSNs.
         // Also, verify if the layer map can be split by drawing a horizontal line at every LSN start/end split point.
         let mut lsn_split_point = BTreeSet::new(); // TODO: use a better data structure (range tree / range set?)
@@ -1820,7 +1822,12 @@ impl Timeline {
                 image_layers.push(layer);
             }
         }
-        let mut merge_iter = MergeIterator::create(&delta_layers, &image_layers, ctx);
+        let (dense_ks, sparse_ks) = self.collect_gc_compaction_keyspace().await?;
+        let mut merge_iter = FilterIterator::create(
+            MergeIterator::create(&delta_layers, &image_layers, ctx),
+            dense_ks,
+            sparse_ks,
+        )?;
         // Step 2: Produce images+deltas. TODO: ensure newly-produced delta does not overlap with other deltas.
         // Data of the same key.
         let mut accumulated_values = Vec::new();