Re: Separated commit log directory configuration

[Nate McCall]

> 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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Separated commit log directory configuration

[Jonathan Ballet]

Hi,

Cassandra's documentation has several recommendations for moving the commit
log directory to a dedicated disk, separated from the sstables disk(s).
However, I couldn't find much information on what would be good practices
regarding this dedicated commit log disk and I'm wondering how to configure
it the best?

We have a Cassandra cluster running 3.11.1 with separate disks for
sstables/commit logs which worked great until a few weeks, but since a
couple of weeks, some nodes sometimes stop working due to a disk full error:

* The error is: "org.apache.cassandra.io.FSWriteError:
java.nio.file.FileSystemException:
/var/lib/cassandra/commitlog/CommitLog-6-1532888559853.log: No space left
on device"
* we have a dedicated disk of 20 GB
* our commit-related settings are as follow (mostly default):

  commit_failure_policy: stop
  commitlog_directory: /var/lib/cassandra/commitlog
  commitlog_segment_size_in_mb: 32
  commitlog_sync: periodic
  commitlog_sync_period_in_ms: 1
  commitlog_total_space_in_mb: 19000

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?)
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?

Thanks!

 Jonathan