[skip ci]Add dropcollection design doc (#11450)

See also: #11426

Signed-off-by: yangxuan <xuan.yang@zilliz.com>

docs/design_docs/drop_collection_release_resources_en.md

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+# DropCollection release resources
+
+## Before this enhancement
+
+**When dropping a collection**
+
+1. DataNode releases the flowgraph of this collection and drops all the data in 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
+
+DN
+- Flush all segment in this vChannel, When Flush&Drop, set the `dropped` flag true.
+    - If fail, retry at most 10 times and restart
+
+DC
+- 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. Easiest approch in both DN and DC
+    2. DN can reuse the current flush manager procedure
+Cons:
+    1. The No. rpc call is equal to the No. segments in a collection, expensive
+
+---
+
+**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 DN.
+Cons:
+    1. More efforts in DN and DC.
+
+```
+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;
+}
+```
+
+---
+
+## Object2: DataCoord 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, remove the binlogs recorded if they've been dropped by 1 day.
+- DataCoord keeps the etcd segmentInfo meta.