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Appendix C. Machine Tuning
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## Appendix C. Machine Tuning

The default tuning parameters available for the Oracle NoSQL Database software should in general be acceptable for production systems, and so do not require any tuning. However, the underlying operating system will have default values for various kernel parameters which require modification in order to achieve the best possible performance for your store's installation.

This appendix identifies the kernel parameters that you should manage when installing a production store. By this, we mean any store whose performance is considered critical. Evaluation systems installed into a lab environment probably do not need this level of tuning unless you are using those systems to measure the store's performance.

### Note

Oracle NoSQL Database is most frequently installed on Linux systems, and so that is what this appendix focuses on.

## Linux Page Cache Tuning

Tune your page cache to permit the OS to write asynchronously to disk whenever possible. This allows background writes, which minimize the latency resulting from serial write operations such as fsync. This also helps with write stalls which occur when the file system cache is full and needs to be flushed to disk to make room for new writes. We have observed significant speedups (15-20%) on insert-intensive benchmarks when these parameters are tuned as described below.

Place the following commands in `/etc/sysctl.conf`. Run

    sysctl -p

to load the new settings so they can take effect without needing to reboot the machine.

    # Set vm.dirty_background_bytes to 10MB to ensure that
    # on a 40MB/sec hard disk a fsync never takes more than 250ms and takes
    # just 125ms on average. The value of vm.dirty_background_bytes
    # should be increased on faster SSDs or I/O subsytems with higher
    # throughput. You should increase this setting by the same proportion
    # as the relative increase in throughput. For example, for a typical SSD
    # with a throughput of 160MB/sec, vm.dirty_background_bytes should be set
    # to 40MB so fsync takes ~250ms. In this case, the value was increased by
    # a factor of 4.
    vm.dirty_background_bytes=10485760

    # IO calls effectively become synchronous(waiting for the underlying
    # device to complete them). This setting helps minimize the
    # possibility of a write request stalling in JE while holding the
    # write log latch.
    vm.dirty_ratio=40

    # Ensures that data does not hang around in memory longer than
    # necessary. Given JE's append-only style of writing, there is
    # typically little benefit from having an intermediate dirty page
    # hanging around, because it is never going to be modified. By
    # evicting the dirty page earlier, its associated memory is readily
    # available for reading or writing new pages, should that become
    # necessary.
    vm.dirty_expire_centisecs=1000

Earlier versions of the Linux kernel may not support `vm.dirty_background_bytes`. On these older kernels you can use `vm.dirty_background_ratio` instead. Pick the ratio that gets you closest to 10MB. On some systems with a lot of memory this may not be possible due to the large granularity associated with this configuration knob. A further impediment is that a ratio of 5 is the effective minimum in some kernels.

    vm.dirty_background_ratio=5

Use `sysctl -a` to verify that the parameters described here are set as expected.

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