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From Arya Asemanfar <>
Subject Soliciting thoughts on possible read optimization
Date Wed, 11 Aug 2010 03:43:58 GMT
I mentioned this today to a couple folks at Cassandra Summit, and thought
I'd solicit some more thoughts here.

Currently, the read stage includes checking row cache. So if your concurrent
reads is N and you have N reads reading from disk, the next read will block
until a disk read finishes, even if it's in row cache. Would it make sense
to isolate disk reads from cache reads? To either make the read stage be
only used on misses, or to make 2 read stages CacheRead and DiskRead? Of
course, we'd have to go to DiskRead for mmap since we wouldn't know until we
asked the OS.

My thought is that stages should be based on resources rather than
semantics, but that may be wrong. Logically, I don't think it would make
sense to have the read stage bounded in a hypothetical system where there is
no IO; it's most likely because of the disk and subsequent IO contention
that that cap was introduced.

As a possible bonus with this change, you can make other optimizations like
batching row reads from disk where the keys were in key cache (does this
even make sense? I'm not too sure how that would work).

Let me know what you guys think.


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