======================== SHEC erasure code plugin ======================== The *shec* plugin encapsulates the `multiple SHEC `_ library. It allows ceph to recover data more efficiently than Reed Solomon codes. Create an SHEC profile ====================== To create a new *shec* erasure code profile:: ceph osd erasure-code-profile set {name} \ plugin=shec \ [k={data-chunks}] \ [m={coding-chunks}] \ [c={durability-estimator}] \ [crush-root={root}] \ [crush-failure-domain={bucket-type}] \ [crush-device-class={device-class}] \ [directory={directory}] \ [--force] Where: ``k={data-chunks}`` :Description: Each object is split in **data-chunks** parts, each stored on a different OSD. :Type: Integer :Required: No. :Default: 4 ``m={coding-chunks}`` :Description: Compute **coding-chunks** for each object and store them on different OSDs. The number of **coding-chunks** does not necessarily equal the number of OSDs that can be down without losing data. :Type: Integer :Required: No. :Default: 3 ``c={durability-estimator}`` :Description: The number of parity chunks each of which includes each data chunk in its calculation range. The number is used as a **durability estimator**. For instance, if c=2, 2 OSDs can be down without losing data. :Type: Integer :Required: No. :Default: 2 ``crush-root={root}`` :Description: The name of the crush bucket used for the first step of the CRUSH rule. For intance **step take default**. :Type: String :Required: No. :Default: default ``crush-failure-domain={bucket-type}`` :Description: Ensure that no two chunks are in a bucket with the same failure domain. For instance, if the failure domain is **host** no two chunks will be stored on the same host. It is used to create a CRUSH rule step such as **step chooseleaf host**. :Type: String :Required: No. :Default: host ``crush-device-class={device-class}`` :Description: Restrict placement to devices of a specific class (e.g., ``ssd`` or ``hdd``), using the crush device class names in the CRUSH map. :Type: String :Required: No. :Default: ``directory={directory}`` :Description: Set the **directory** name from which the erasure code plugin is loaded. :Type: String :Required: No. :Default: /usr/lib/ceph/erasure-code ``--force`` :Description: Override an existing profile by the same name. :Type: String :Required: No. Brief description of SHEC's layouts =================================== Space Efficiency ---------------- Space efficiency is a ratio of data chunks to all ones in a object and represented as k/(k+m). In order to improve space efficiency, you should increase k or decrease m. :: space efficiency of SHEC(4,3,2) = 4/(4+3) = 0.57 SHEC(5,3,2) or SHEC(4,2,2) improves SHEC(4,3,2)'s space efficiency Durability ---------- The third parameter of SHEC (=c) is a durability estimator, which approximates the number of OSDs that can be down without losing data. ``durability estimator of SHEC(4,3,2) = 2`` Recovery Efficiency ------------------- Describing calculation of recovery efficiency is beyond the scope of this document, but at least increasing m without increasing c achieves improvement of recovery efficiency. (However, we must pay attention to the sacrifice of space efficiency in this case.) ``SHEC(4,2,2) -> SHEC(4,3,2) : achieves improvement of recovery efficiency`` Erasure code profile examples ============================= :: $ ceph osd erasure-code-profile set SHECprofile \ plugin=shec \ k=8 m=4 c=3 \ crush-failure-domain=host $ ceph osd pool create shecpool 256 256 erasure SHECprofile