问题
How is the behavior of memory_only and memory_and_disk caching level in spark differ?
回答1:
Documentation says ---
Storage Level
Meaning
MEMORY_ONLY
Store RDD as deserialized Java objects in the JVM. If the RDD does not fit in memory, some partitions will not be cached and will be recomputed on the fly each time they're needed. This is the default level.
MEMORY_AND_DISK
Store RDD as deserialized Java objects in the JVM. If the RDD does not fit in memory, store the partitions that don't fit on disk, and read them from there when they're needed.
MEMORY_ONLY_SER
Store RDD as serialized Java objects (one byte array per partition). This is generally more space-efficient than deserialized objects, especially when using a fast serializer, but more CPU-intensive to read.
MEMORY_AND_DISK_SER
Similar to MEMORY_ONLY_SER, but spill partitions that don't fit in memory to disk instead of recomputing them on the fly each time they're needed.
DISK_ONLY
Store the RDD partitions only on disk.
MEMORY_ONLY_2, MEMORY_AND_DISK_2, etc.
Same as the levels above, but replicate each partition on two cluster nodes.
OFF_HEAP (experimental)
Store RDD in serialized format in Tachyon. Compared to MEMORY_ONLY_SER, OFF_HEAP reduces garbage collection overhead and allows executors to be smaller and to share a pool of memory, making it attractive in environments with large heaps or multiple concurrent applications. Furthermore, as the RDDs reside in Tachyon, the crash of an executor does not lead to losing the in-memory cache. In this mode, the memory in Tachyon is discardable. Thus, Tachyon does not attempt to reconstruct a block that it evicts from memory.
It means for Memory ONLY, spark will try to keep partitions in memory always. If some partitions can not be kept in memory, or for node loss some partitions are removed from RAM, spark will recompute using lineage information. In memory-and-disk level, spark will always keep partitions computed and cached. It will try to keep in RAM, but if it does not fit then paritions will be spilled to disk.
回答2:
As explained in the documentation, Persistence levels in terms of efficiency:
Level Space used CPU time In memory On disk Serialized ------------------------------------------------------------------------- MEMORY_ONLY High Low Y N N MEMORY_ONLY_SER Low High Y N Y MEMORY_AND_DISK High Medium Some Some Some MEMORY_AND_DISK_SER Low High Some Some Y DISK_ONLY Low High N Y Y
MEMORY_AND_DISK and MEMORY_AND_DISK_SER spill to disk if there is too much data to fit in memory.
来源：https://stackoverflow.com/questions/30520428/what-is-the-difference-between-memory-only-and-memory-and-disk-caching-level-in