milvus/docs/design_docs/20211224-drop_collection_release_resources.md

DropCollection release resources

Before this enhancement

When dropping a collection

1. DataNode releases the flowgraph of this collection and drops all the data in a buffer.
2. DataCoord has no idea whether a collection is dropped or not.
   • DataCoord will make DataNode watch DmChannels of dropped collections.
   • Blob files will never be removed even if the collection is dropped.

For not in used binlogs on blob storage: Why are there such binlogs

• A failure flush.
• A failure compaction.
• Dropped and out-of timetravel collection binlogs.

This enhancement is focused on solving these 2 problems.

Object1 DropCollection

DataNode ignites Flush&Drop receive drop collection msg -> cancel compaction -> flush all insert buffer and delete buffer -> release the flowgraph

Plan 1: Picked

Add a dropped flag in SaveBinlogPathRequest proto.

DataNode

• Flush all segments in this vChannel, When Flush&Drop, set the dropped flag true.
  • If fails, retry at most 10 times and restart.

DataCoord

• DataCoord marks segmentInfo as dropped, doesn't remove segmentInfos from etcd.
• When recovery, check if the segments in the vchannel are all dropped.
  • if not, recover before the drop.
  • if so, no need to recover the vchannel.

Pros: 1. The easiest approach in both DataNode and DataCoord. 2. DataNode can reuse the current flush manager procedure. Cons: 1. The No. rpc call is equal to the No. segments in a collection, expensive.

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Plan 2: Enhance later

Add a new rpc FlushAndDrop, it's a vchannel scope rpc.

Pros: 1. much lesser rpc calls, equal to shard-numbers. 2. More clarity of flush procedure in DataNode. Cons: 1. More efforts in DataNode and DataCoord.

message FlushAndDropRequest {
  common.MsgBase base = 1;
  string channelID = 2;
  int64 collectionID = 3;
  repeated SegmentBinlogPaths segment_binlog_paths = 6;
}

message SegmentBinlogPaths {
  int64 segmentID = 1;
  CheckPoint checkPoint = 2;
  repeated FieldBinlog field2BinlogPaths = 2;
  repeated FieldBinlog field2StatslogPaths = 3;
  repeated DeltaLogInfo deltalogs = 4;
}

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Object2: DataCoord Garbage Collection (GC) for not in used binlogs

How to clear unknown binlogs?

DataCoord runs a background GC goroutine, triggers every 1 day:

1. Get all minIO/S3 paths(keys).
2. Filter out keys not in segmentInfo.
3. According to the meta of blobs from minIO/S3, remove binlogs that exist more than 1 day.
   • **Why 1 day: **Maybe there are newly uploaded binlogs from flush/compaction

How to clear dropped-collection's binlogs?

• DataCoord checks all dropped-segments, removes the binlogs recorded if they've been dropped by 1 day.
• DataCoord keeps the etcd segmentInfo meta.