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From Jonathan Guberman ...@tineye.com>
Subject Cassandra as a key/object store for many small (10-60k) files
Date Fri, 05 May 2017 19:19:44 GMT
Hello,

We’re currently testing Cassandra for use as a pure key-object store for data blobs around
10kB - 60kB each. Our use case is storing on the order of 10 billion objects with about 5-20
million new writes per day. A written object will never be updated or deleted. Objects will
be read at least once, some time within 10 days of being written. This will generally happen
as a batch; that is, all of the images written on a particular day will be read together at
the same time. This batch read will only happen one time; future reads will happen on individual
objects, with no grouping, and they will follow a long-tail distribution, with popular objects
read thousands of times per year but most read never or virtually never.

I’ve set up a small four node test cluster and have written test scripts to benchmark writing
and reading our data. The table I’ve set up is very simple: an ascii primary key column
with the object ID and a blob column for the data. All other settings were left at their defaults.
 
I’ve found write speeds to be very fast most of the time. However, periodically, writes
will slow to a crawl for anywhere between half an hour to two hours, after which speeds recover
to their previous levels. I assume this is some sort of data compaction or flushing to disk,
but I haven’t been able to figure out the exact cause.

Read speeds have been more disappointing. Cached reads are very fast, but random read speed
averages about 2 MB/sec, which is too slow when we need to read out a batch of several million
objects. I don’t think it’s reasonable to assume that these rows will all still be cached
by the time we need to read them for that first large batch read.

My general question is whether anyone has any suggestions for how to improve performance for
our use case. More specifically:

- Is there a way to mitigate or eliminate the huge slowdowns I see when writing millions of
rows?
- Are there settings I should be using in order to maximize read speeds for random reads?
- Is there a way to design our tables to improve the read speeds for the initial large batched
reads? I was thinking of using a batch ID column that could be used to retrieve the data for
the initial block. However, future reads would need to be done by the object ID, not the batch
ID, so it seems to me I’d need to duplicate the data, one in a “objects by batch” table,
and the other in a simple “objects” table. Is there a better approach than this?

Thank you!



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