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From Nate McCall <zznat...@gmail.com>
Subject Re: Separated commit log directory configuration
Date Mon, 01 Oct 2018 00:43:32 GMT
> We only increased commitlog_total_space_in_mb so that Cassandra fully uses the dedicated
disk, but that may be an error?
> The default value for this setting is (per the documentation):
>
>     The default value is the smaller of 8192, and 1/4 of the total space of the commitlog
volume.
>
> But that doesn't say much  (or should it really by 25% of the disk space?)

I wouldnt fill any filesystem intentionally that close to its
partition size. Most (all?) will start to degrade performance-wise.
Unless you are really strapped for disk space, give it some breathing
room. This is best chosen by monitoring commitlog rotation frequency
in conjunction with disk utilization for your cluster.

>
> So, my questions would be:
>
> * What size should I dedicate to this commit log disk? What are the rules of thumb to
discover the "best" size?
> * How should I configure the "commitlog_total_space_in_mb" setting respectively to the
size of the disk?

Most clusters shouldnt need to adjust this or any of the default
commitlog settings unless you have excessively large mutations or
require the commitlog getting written to disk more frequently.

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