[SPARK-2275] More general Storage Interface for Shuffle / Spill etc.

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Done
• Component/s: Block Manager, Shuffle, Spark Core
• Assignee: Unassigned
• Reporter: Raymond Liu
• Created: 25/Jun/14 07:26
• Updated: 17/May/20 18:21
• Resolved: 04/Sep/14 01:09

Description

Problem 1:

In the current design, when shuffle / spill is involved, A File based interface is assumed for various classes. While this might not been true when we have disk store implemented base on something else other than FileSystem ( e.g. an kv/object based NVM device) . And also if we try to utilize memory or off heap store for shuffle, the File interface also do not work.

Possible approaching :

So my general idea here is to hide the File Interface, instead using a general ObjectId to represent the object been written to external store, And pass around this ObjectId to various class to access the data.

e.g.

For Write path: DiskBlockObjectWritter( could be rename to FileBlockObjectWritter) now take in ObjectId instead of File and do mapping to File internally

For Read path , A InputStream Interface is suppose to be able to retrieved from the ObjectId by specific store, Thus various read operation do not need to rely on the assumption that the lower level storage is a filesystem and rely on File to build their own FileInputStream etc.

In this way, the current File base diskStore could still using File to implement it's internal storage, while other solution could be easily been plug in with other low level implementation and just mapping to the ObjectId for other module to interact with.

Problem 2 :

At present, In shuffle write path, the shuffle block manager manage the mapping from some blockID to a FileSegment for the benefit of consolidate shuffle, this way it bypass the block store's blockId based access mode. Then in the read path, when read a shuffle block data, disk store query shuffleBlockManager to hack the normal blockId to file mapping in order to correctly read data from file. This really rend to a lot of bi-directional dependencies between modules and the code logic is some how messed up. None of the shuffle block manager and blockManager/Disk Store fully control the read path. They are tightly coupled in low level code modules. And it make it hard to implement other shuffle manager logics. e.g. a sort based shuffle which might merge all output from one map partition to a single file. This will need to hack more into the diskStore/diskBlockManager etc to find out the right data to be read.

Possible approaching:

So I think it might be better that we expose an object + offset based read interface for BlockStore, ( or at least for DiskStore), e.g. a getObjectData(objectId, offset, length) in addition to the current blockID based interface.

Then those mapping blockId to object and offset code logic can all reside in the specific shuffle manager, if they do need to merge data into one single object(File here in current diskStore implementation) they take care of the mapping logic in both read/write path and take the responsibility of read / write shuffle data ( since they already take care of write data, then read data also go through them instead of go through blockmanager is also reasonable, they can further use the object+offset based read interface for actual read work )

The BlockStore itself should just take care of read/write as required, it should not involve into the data mapping logic at all. This might make the interface between modules more clear and decouple each other in a more clean way.

Issue Links

• is related to: SPARK-1733 Pluggable storage support for BlockManager
• is related to: SPARK-2044 Pluggable interface for shuffles

Sub-Tasks

• Hide ShuffleBlockManager behind ShuffleManager: Resolved