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[ceph.git] / ceph / src / rocksdb / include / rocksdb / compaction_filter.h

diff --git a/ceph/src/rocksdb/include/rocksdb/compaction_filter.h b/ceph/src/rocksdb/include/rocksdb/compaction_filter.h
index 98f86c2812083fa613382f20d111e7be0e605c78..5d476fb8e6efd74a5643e819e31eb5e1f79ca0ab 100644 (file)
--- a/ceph/src/rocksdb/include/rocksdb/compaction_filter.h
+++ b/ceph/src/rocksdb/include/rocksdb/compaction_filter.h
@@ -75,14 +75,11 @@ class CompactionFilter {
   // to modify the existing_value and pass it back through new_value.
   // value_changed needs to be set to true in this case.
   //
-  // If you use snapshot feature of RocksDB (i.e. call GetSnapshot() API on a
-  // DB* object), CompactionFilter might not be very useful for you. Due to
-  // guarantees we need to maintain, compaction process will not call Filter()
-  // on any keys that were written before the latest snapshot. In other words,
-  // compaction will only call Filter() on keys written after your most recent
-  // call to GetSnapshot(). In most cases, Filter() will not be called very
-  // often. This is something we're fixing. See the discussion at:
-  // https://www.facebook.com/groups/mysqlonrocksdb/permalink/999723240091865/
+  // Note that RocksDB snapshots (i.e. call GetSnapshot() API on a
+  // DB* object) will not guarantee to preserve the state of the DB with
+  // CompactionFilter. Data seen from a snapshot might disppear after a
+  // compaction finishes. If you use snapshots, think twice about whether you
+  // want to use compaction filter and whether you are using it in a safe way.
   //
   // If multithreaded compaction is being used *and* a single CompactionFilter
   // instance was supplied via Options::compaction_filter, this method may be
@@ -135,9 +132,9 @@ class CompactionFilter {
   //
   //      Caveats:
   //       - The keys are skipped even if there are snapshots containing them,
-  //         as if IgnoreSnapshots() was true; i.e. values removed
-  //         by kRemoveAndSkipUntil can disappear from a snapshot - beware
-  //         if you're using TransactionDB or DB::GetSnapshot().
+  //         i.e. values removed by kRemoveAndSkipUntil can disappear from a
+  //         snapshot - beware if you're using TransactionDB or
+  //         DB::GetSnapshot().
   //       - If value for a key was overwritten or merged into (multiple Put()s
   //         or Merge()s), and compaction filter skips this key with
   //         kRemoveAndSkipUntil, it's possible that it will remove only
@@ -176,15 +173,12 @@ class CompactionFilter {
     return Decision::kKeep;
   }
 
-  // By default, compaction will only call Filter() on keys written after the
-  // most recent call to GetSnapshot(). However, if the compaction filter
-  // overrides IgnoreSnapshots to make it return true, the compaction filter
-  // will be called even if the keys were written before the last snapshot.
-  // This behavior is to be used only when we want to delete a set of keys
-  // irrespective of snapshots. In particular, care should be taken
-  // to understand that the values of these keys will change even if we are
-  // using a snapshot.
-  virtual bool IgnoreSnapshots() const { return false; }
+  // This function is deprecated. Snapshots will always be ignored for
+  // compaction filters, because we realized that not ignoring snapshots doesn't
+  // provide the gurantee we initially thought it would provide. Repeatable
+  // reads will not be guaranteed anyway. If you override the function and
+  // returns false, we will fail the compaction.
+  virtual bool IgnoreSnapshots() const { return true; }
 
   // Returns a name that identifies this compaction filter.
   // The name will be printed to LOG file on start up for diagnosis.
@@ -195,7 +189,7 @@ class CompactionFilter {
 // application to know about different compactions
 class CompactionFilterFactory {
  public:
-  virtual ~CompactionFilterFactory() { }
+  virtual ~CompactionFilterFactory() {}
 
   virtual std::unique_ptr<CompactionFilter> CreateCompactionFilter(
       const CompactionFilter::Context& context) = 0;