[Feature] Hybrid Compression

[sarthakaggarwal97]

Is your feature request related to a problem? Please describe

In Lucene, we have Stored Fields. The text in the such fields is stored in the index literally, in a non-inverted manner.

OpenSearch, by default, makes the _source field of an index as stored. The users have an option to store other fields of the document as well. These fields are directly compressed and stored in the .fdt file of the segment. We use Index Codecs to determine the compression algorithm that will be used for compress and decompress these stored fields. The compression of these fields in the write path is dependent on two conditions: Chunk Size and the Number of Documents.

With Hybrid Compression, we would take the compression off the write path and would store the data as is in the segments. During merges, when the segment size have reached a certain threshold, then we would compress the segments.

We are looking to save up on compute of compression during writes to improve latency and throughput with a trade off on disk.

Describe the solution you'd like

How to decide when we would perform compression? During the initialization of merges between segments, the Lucene estimates the size of each to be merged segment as estimatedMergeBytes. We will leverage this value in the SegmentInfo and use our own set thresholds to compare with estimatedMergeBytes. In OpenSearch, we would initiate the compression once the segments have breached these thresholds. These thresholds would be dynamically configurable with an index setting.

This is POC implementation of the change in OpenSearch. Since we would be required to create new index codecs, we can direct this change to custom-codecs as well. Since we do not have estimatedMergeBytes in the SegmentInfos, we would see a change in lucene as well.

What are the cases where users would Hybrid Compression the most? It is expected that hybrid compression would be useful for search and update use cases, specially when the users access or update recently indexed data since we will save up compression / decompression compute.

Benchmarks: Workload: NYC Taxis We tested three hybrid compression size thesholds: 16mb, 32mb and 64mb. The results are of 64mb (looked to be best amongst others) Operation	Codec	Refresh Interval	Disk Throughput Configured	Throughput	Latency	Write IOPS
Index	default	1s	593 mb/s	4.50%	5.50%	-11%
	default	30s	593 mb/s	4.20%	5%	-220%
	default	30s	250 mb/s	6%	14%	-220%

Operation	Codec	Refresh Interval	Disk Throughput Configured	Throughput	Latency	Read IOPS
Update	default	1s	593 mb/s	3.50%	5.50%	-12%
	default	30s	593 mb/s	6	8.50%	-300%
	default	30s	250 mb/s	15%	16%	-300%

Note: +ve means improvement, -ve means degradation from the current behaviour.

Variance in Storage during the indexing of NYC Taxis Workload

There are steeper dips in the storage of the disk but it quickly recovers as we reach the 64 mb segment size thresholds.

Hybrid Compression:

Default Compression

Benchmarking Setup:

1. 3 Dedicated Master Nodes - r6x.xlarge
2. 1 Data Node - r6x.2xlarge
3. EBS Configuration:
   1. Storage: 1000gb
   2. IOPS: 3000
   3. Throughput: 593 mb/s and 250 mb/s

[mgodwan]

@sarthakaggarwal97 We already have an issue to discuss this https://github.com/opensearch-project/OpenSearch/issues/11605

Could we use the same issue to continue discussion?

[peternied]

[Triage - attendees 1 2 3 4 5 6] @sarthakaggarwal97 Thanks for creating this issue, look forward to a pull request to add this functionality

[reta]

@sarthakaggarwal97 any overlap with https://github.com/opensearch-project/OpenSearch/issues/12948 which suggest to completely offload _source storage? How beneficial the hybrid compression will be in this case?

[sarthakaggarwal97]

@reta yeah, we are working on optimizing the flows for stored fields. Currently, _source is by default the only stored field in OpenSearch. So if do offload this field (however it seems like a long effort considering things like update, deletes, reindex etc needs to be thought through), we might see minimal benefit, only for the users who specifically mark their fields as stored in the document.