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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6809) Compressed Commit Log
Date Wed, 20 Aug 2014 09:17:27 GMT


Benedict commented on CASSANDRA-6809:

bq. Thus, the main CL thread(s) can continue processing the new entries and writing to the
mmap buffer,

I think you're still thinking in 2.0 terms. In 2.1 the mutation threads write to the commit
log buffer directly. We will need these buffers to be purely in memory (not mmapped) and then
passed to the sync threads when ready for writing to be compressed and written to disk.

bq. I wasn't thinking about the write perf, necessarily, but about having the file contiguous
on disk

I'm not convinced this will behave any differently performance-wise (i.e. if we were to preallocate
the file's size), however we don't now know the size of the file we'll be writing, so pre-allocating
doesn't really help much anymore.

bq. a simple solution might be to have a sync thread that merely invokes the mmap buffer flush

But this flush is synchronous? Specifically, we want multiple to be in flight at once. I also
do not think there's any benefit to using mmap any more, since you bring it up. We should
switch to regular output streams, so that we can simply wrap it in a compressed output stream.
If you mean to start using async IO in Java, this isn't really any superior here - it still
involves extra threads with a hidden thread pool, and extra (IMO) complexity beyond simply
performing it on the relevant thread, but also would involve compressing the entire segment
before writing to the file, which introduces extra latency  (or having a complex to-and-fro
shuttle compressed data to an async sink, waiting for result, etc), for no benefit.

> Compressed Commit Log
> ---------------------
>                 Key: CASSANDRA-6809
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Benedict
>            Assignee: Branimir Lambov
>            Priority: Minor
>              Labels: performance
>             Fix For: 3.0
> It seems an unnecessary oversight that we don't compress the commit log. Doing so should
improve throughput, but some care will need to be taken to ensure we use as much of a segment
as possible. I propose decoupling the writing of the records from the segments. Basically
write into a (queue of) DirectByteBuffer, and have the sync thread compress, say, ~64K chunks
every X MB written to the CL (where X is ordinarily CLS size), and then pack as many of the
compressed chunks into a CLS as possible.

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