milvus/docs/design_docs/segcore/segment_growing.md

SegmentGrowing

Growing segment has the following additional interfaces:

1. PreInsert(size) -> reseveredOffset: serial interface, which reserves space for future insertion and returns the reseveredOffset.

2. Insert(reseveredOffset, size, ...Data...): write ...Data... into range [reseveredOffset, reseveredOffset + size). This interface is allowed to be called concurrently.

   1. ...Data... contains row_ids, timestamps two system attributes, and other columns
   2. data columns can be stored either row-based or column-based.
   3. PreDelete & Delete(reseveredOffset, row_ids, timestamps) is a delete interface similar to insert interface.

Growing segment stores data in the form of chunk. The number of rows in each chunk is restricted by configs.

Rows per segment are controlled by parameters size_per_Chunk config

When inserting, first allocate enough space to ensure total_size <= num_chunk * size_per_chunk, and then convert data from row format to column format.

During a search, each 'chunk' will be searched, and the search results will be saved as 'subquery result', then reduced into TopK.

Growing Segment also implements small batch index for vectors. The parameters of small batch index are preset in segcore config

When metric type is specified in the schema, the default parameters will build index for each chunk to accelerate query

SegmentGrowingImpl internal

1. SegcoreConfig: contains parameters for Segcore，it has to be specified before create segment
2. InsertRecord: inserted data put to here
3. DeleteRecord: wait for delete implementation
4. IndexingRecord: contains data with small index
5. SealedIndexing: Record not used any more

SegcoreConfig

1. Manage chunk_sizeand small index parameters
2. parse_from can parse from yaml files（this function is not enabled by default）
   • refer to ${milvus}/internal/core/unittest/test_utils/test_segcore.yaml
3. default_config offers default parameters

InsertRecord

Used to manage concurrent inserted data, including:

1. atomic<int64_t> reserved reserved space calculation
2. AckResponder calculate which segment to insert, returns current segment offset
3. ConcurrentVector stores data columns, each column has one concurrent vector

The following steps are executed when insert,

1. Serially Execute PreInsert(size) -> reserved_offset to allocate memory space, the address of space is [reserved_offset, reserved_offset + size) is reserved

2. Parallelly execute Insert(reserved_offset, size, ...Data...) interface，copy data into the above memory address

   • First of all，for ConcurrentVector of each column, call grow_to_at_least to reserve space
   • For each column data, call set_data_raw interface to put data into corresponding locations.
   • After execution finished，callAddSegment of AckResponder ，mark the space [reserved_offset, reserved_offset + size) to already inserted

ConcurrentVector

This is a column data storage that can be inserted concurrently. It is composed of multi-data chunks.

1. Aftergrow_to_at_least(size) called, reserve space no less than size
2. set_data_raw(element_offset, source, element_count) point source to continuous piece of data
3. get_span(chunk_id) get the span of the corresponding chunk