Currently when we read a compressed sstable we copy the data on heap then send it to be de-compressed to another on heap buffer (albeit pooled).
But now both snappy and lz4 (with
CASSANDRA-7039) allow decompression of direct byte buffers. This lets us mmap the data and decompress completely off heap (and avoids moving bytes over JNI).
One issue is performing the checksum offheap but the Adler32 does support in java 8 (it's also in java 7 but marked private?!)
This change yields a > 10% boost in read performance on cstar. Locally I see upto 30% improvement.