Bcache Tuning

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• cache_mode | string

  Default: unmanaged

  Can be one of either writethrough, writeback, writearound, none, or unmanaged. The unmanaged setting instructs the charm that the cache mode is managed externally, e.g. via MAAS or manually.

• congested_read_threshold_us | int

  Traffic's still going to the spindle/still getting cache misses . In the real world, SSDs don't always keep up with disks - particularly with slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So you want to avoid being bottlenecked by the SSD and having it slow everything down. . To avoid that bcache tracks latency to the cache device, and gradually throttles traffic if the latency exceeds a threshold (it does this by cranking down the sequential bypass). . You can disable this if you need to by setting the thresholds to 0. . Note that the kernel default is 2000 us (2 milliseconds) for reads.

• congested_write_threshold_us | int

  Traffic's still going to the spindle/still getting cache misses . In the real world, SSDs don't always keep up with disks - particularly with slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So you want to avoid being bottlenecked by the SSD and having it slow everything down. . To avoid that bcache tracks latency to the cache device, and gradually throttles traffic if the latency exceeds a threshold (it does this by cranking down the sequential bypass). . You can disable this if you need to by setting the thresholds to 0. . Note that the kernel default is 20000 us (20 milliseconds) for writes.

• debug | boolean

  Enable debug logging.

• nagios_context | string

  Default: juju

  Used by the nrpe subordinate charms. A string that will be prepended to instance name to set the host name in nagios. So for instance the hostname would be something like: juju-myservice-0 If you're running multiple environments with the same services in them this allows you to differentiate between them.

• nagios_servicegroups | string

  A comma-separated list of nagios servicegroups. If left empty, the nagios_context will be used as the servicegroup

• readahead | int

  Size of readahead that should be performed. Defaults to 0. If set to e.g. 1M, it will round cache miss reads up to that size, but without overlapping existing cache entries.

• sequential_cutoff | int

  A sequential IO will bypass the cache once it passes this threshhold; the most recent 128 IOs are tracked so sequential IO can be detected even when it isn't all done at once.

• writeback_percent | int

  Default: 10

  If nonzero, bcache tries to keep around this percentage of the cache dirty by throttling background writeback and using a PD controller to smoothly adjust the rate.

• writeback_rate_fp_term_factor | int

  Default: 1

  This is a tuning option that will increase the value of writeback_rate_fp_term_{low|mid|high} by this factor . This is useful to optimize the rate when one cache device is shared by multiple backing devices, especially useful when those backing devices have the same size and similar workloads, e.g. OSD node . Suggesting set this value to the number of backing devices.