Considerations for using a RAID controller with OSD hosts

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Use this information when planning for RAID controller usage with OSD hosts.

  • If an OSD host has a RAID controller with 1 - 2 Gb of cache installed, enabling the write-back cache might result in increased small I/O write throughput. However, the cache must be nonvolatile.

  • Most modern RAID controllers have super capacitors that provide enough power to drain volatile memory to nonvolatile NAND memory during a power-loss event. It is important to understand how a particular controller and its firmware behave after power is restored.

  • Some RAID controllers require manual intervention. Hard disks typically advertise to the operating system whether their disk caches can be enabled or disabled by default. However, certain RAID controllers and some firmware do not provide such information. Verify that disk level caches are disabled to avoid file system corruption.

  • Create a single RAID 0 volume with write-back for each Ceph OSD data drive with write-back cache enabled.

  • If Serial Attached SCSI (SAS) or SATA connected Solid-state Drive (SSD) disks are also present on the RAID controller, then investigate whether the controller and firmware support pass-through mode. Enabling pass-through mode helps avoid caching logic, and generally results in lower latency for fast media.