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diff --git a/src/ceph/doc/rados/operations/erasure-code-lrc.rst b/src/ceph/doc/rados/operations/erasure-code-lrc.rst new file mode 100644 index 0000000..447ce23 --- /dev/null +++ b/src/ceph/doc/rados/operations/erasure-code-lrc.rst @@ -0,0 +1,371 @@ +====================================== +Locally repairable erasure code plugin +====================================== + +With the *jerasure* plugin, when an erasure coded object is stored on +multiple OSDs, recovering from the loss of one OSD requires reading +from all the others. For instance if *jerasure* is configured with +*k=8* and *m=4*, losing one OSD requires reading from the eleven +others to repair. + +The *lrc* erasure code plugin creates local parity chunks to be able +to recover using less OSDs. For instance if *lrc* is configured with +*k=8*, *m=4* and *l=4*, it will create an additional parity chunk for +every four OSDs. When a single OSD is lost, it can be recovered with +only four OSDs instead of eleven. + +Erasure code profile examples +============================= + +Reduce recovery bandwidth between hosts +--------------------------------------- + +Although it is probably not an interesting use case when all hosts are +connected to the same switch, reduced bandwidth usage can actually be +observed.:: + + $ ceph osd erasure-code-profile set LRCprofile \ + plugin=lrc \ + k=4 m=2 l=3 \ + crush-failure-domain=host + $ ceph osd pool create lrcpool 12 12 erasure LRCprofile + + +Reduce recovery bandwidth between racks +--------------------------------------- + +In Firefly the reduced bandwidth will only be observed if the primary +OSD is in the same rack as the lost chunk.:: + + $ ceph osd erasure-code-profile set LRCprofile \ + plugin=lrc \ + k=4 m=2 l=3 \ + crush-locality=rack \ + crush-failure-domain=host + $ ceph osd pool create lrcpool 12 12 erasure LRCprofile + + +Create an lrc profile +===================== + +To create a new lrc erasure code profile:: + + ceph osd erasure-code-profile set {name} \ + plugin=lrc \ + k={data-chunks} \ + m={coding-chunks} \ + l={locality} \ + [crush-root={root}] \ + [crush-locality={bucket-type}] \ + [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: Yes. +:Example: 4 + +``m={coding-chunks}`` + +:Description: Compute **coding chunks** for each object and store them + on different OSDs. The number of coding chunks is also + the number of OSDs that can be down without losing data. + +:Type: Integer +:Required: Yes. +:Example: 2 + +``l={locality}`` + +:Description: Group the coding and data chunks into sets of size + **locality**. For instance, for **k=4** and **m=2**, + when **locality=3** two groups of three are created. + Each set can be recovered without reading chunks + from another set. + +:Type: Integer +:Required: Yes. +:Example: 3 + +``crush-root={root}`` + +:Description: The name of the crush bucket used for the first step of + the ruleset. For intance **step take default**. + +:Type: String +:Required: No. +:Default: default + +``crush-locality={bucket-type}`` + +:Description: The type of the crush bucket in which each set of chunks + defined by **l** will be stored. For instance, if it is + set to **rack**, each group of **l** chunks will be + placed in a different rack. It is used to create a + ruleset step such as **step choose rack**. If it is not + set, no such grouping is done. + +:Type: String +:Required: No. + +``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 ruleset 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. + +Low level plugin configuration +============================== + +The sum of **k** and **m** must be a multiple of the **l** parameter. +The low level configuration parameters do not impose such a +restriction and it may be more convienient to use it for specific +purposes. It is for instance possible to define two groups, one with 4 +chunks and another with 3 chunks. It is also possible to recursively +define locality sets, for instance datacenters and racks into +datacenters. The **k/m/l** are implemented by generating a low level +configuration. + +The *lrc* erasure code plugin recursively applies erasure code +techniques so that recovering from the loss of some chunks only +requires a subset of the available chunks, most of the time. + +For instance, when three coding steps are described as:: + + chunk nr 01234567 + step 1 _cDD_cDD + step 2 cDDD____ + step 3 ____cDDD + +where *c* are coding chunks calculated from the data chunks *D*, the +loss of chunk *7* can be recovered with the last four chunks. And the +loss of chunk *2* chunk can be recovered with the first four +chunks. + +Erasure code profile examples using low level configuration +=========================================================== + +Minimal testing +--------------- + +It is strictly equivalent to using the default erasure code profile. The *DD* +implies *K=2*, the *c* implies *M=1* and the *jerasure* plugin is used +by default.:: + + $ ceph osd erasure-code-profile set LRCprofile \ + plugin=lrc \ + mapping=DD_ \ + layers='[ [ "DDc", "" ] ]' + $ ceph osd pool create lrcpool 12 12 erasure LRCprofile + +Reduce recovery bandwidth between hosts +--------------------------------------- + +Although it is probably not an interesting use case when all hosts are +connected to the same switch, reduced bandwidth usage can actually be +observed. It is equivalent to **k=4**, **m=2** and **l=3** although +the layout of the chunks is different:: + + $ ceph osd erasure-code-profile set LRCprofile \ + plugin=lrc \ + mapping=__DD__DD \ + layers='[ + [ "_cDD_cDD", "" ], + [ "cDDD____", "" ], + [ "____cDDD", "" ], + ]' + $ ceph osd pool create lrcpool 12 12 erasure LRCprofile + + +Reduce recovery bandwidth between racks +--------------------------------------- + +In Firefly the reduced bandwidth will only be observed if the primary +OSD is in the same rack as the lost chunk.:: + + $ ceph osd erasure-code-profile set LRCprofile \ + plugin=lrc \ + mapping=__DD__DD \ + layers='[ + [ "_cDD_cDD", "" ], + [ "cDDD____", "" ], + [ "____cDDD", "" ], + ]' \ + crush-steps='[ + [ "choose", "rack", 2 ], + [ "chooseleaf", "host", 4 ], + ]' + $ ceph osd pool create lrcpool 12 12 erasure LRCprofile + +Testing with different Erasure Code backends +-------------------------------------------- + +LRC now uses jerasure as the default EC backend. It is possible to +specify the EC backend/algorithm on a per layer basis using the low +level configuration. The second argument in layers='[ [ "DDc", "" ] ]' +is actually an erasure code profile to be used for this level. The +example below specifies the ISA backend with the cauchy technique to +be used in the lrcpool.:: + + $ ceph osd erasure-code-profile set LRCprofile \ + plugin=lrc \ + mapping=DD_ \ + layers='[ [ "DDc", "plugin=isa technique=cauchy" ] ]' + $ ceph osd pool create lrcpool 12 12 erasure LRCprofile + +You could also use a different erasure code profile for for each +layer.:: + + $ ceph osd erasure-code-profile set LRCprofile \ + plugin=lrc \ + mapping=__DD__DD \ + layers='[ + [ "_cDD_cDD", "plugin=isa technique=cauchy" ], + [ "cDDD____", "plugin=isa" ], + [ "____cDDD", "plugin=jerasure" ], + ]' + $ ceph osd pool create lrcpool 12 12 erasure LRCprofile + + + +Erasure coding and decoding algorithm +===================================== + +The steps found in the layers description:: + + chunk nr 01234567 + + step 1 _cDD_cDD + step 2 cDDD____ + step 3 ____cDDD + +are applied in order. For instance, if a 4K object is encoded, it will +first go thru *step 1* and be divided in four 1K chunks (the four +uppercase D). They are stored in the chunks 2, 3, 6 and 7, in +order. From these, two coding chunks are calculated (the two lowercase +c). The coding chunks are stored in the chunks 1 and 5, respectively. + +The *step 2* re-uses the content created by *step 1* in a similar +fashion and stores a single coding chunk *c* at position 0. The last four +chunks, marked with an underscore (*_*) for readability, are ignored. + +The *step 3* stores a single coding chunk *c* at position 4. The three +chunks created by *step 1* are used to compute this coding chunk, +i.e. the coding chunk from *step 1* becomes a data chunk in *step 3*. + +If chunk *2* is lost:: + + chunk nr 01234567 + + step 1 _c D_cDD + step 2 cD D____ + step 3 __ _cDDD + +decoding will attempt to recover it by walking the steps in reverse +order: *step 3* then *step 2* and finally *step 1*. + +The *step 3* knows nothing about chunk *2* (i.e. it is an underscore) +and is skipped. + +The coding chunk from *step 2*, stored in chunk *0*, allows it to +recover the content of chunk *2*. There are no more chunks to recover +and the process stops, without considering *step 1*. + +Recovering chunk *2* requires reading chunks *0, 1, 3* and writing +back chunk *2*. + +If chunk *2, 3, 6* are lost:: + + chunk nr 01234567 + + step 1 _c _c D + step 2 cD __ _ + step 3 __ cD D + +The *step 3* can recover the content of chunk *6*:: + + chunk nr 01234567 + + step 1 _c _cDD + step 2 cD ____ + step 3 __ cDDD + +The *step 2* fails to recover and is skipped because there are two +chunks missing (*2, 3*) and it can only recover from one missing +chunk. + +The coding chunk from *step 1*, stored in chunk *1, 5*, allows it to +recover the content of chunk *2, 3*:: + + chunk nr 01234567 + + step 1 _cDD_cDD + step 2 cDDD____ + step 3 ____cDDD + +Controlling crush placement +=========================== + +The default crush ruleset provides OSDs that are on different hosts. For instance:: + + chunk nr 01234567 + + step 1 _cDD_cDD + step 2 cDDD____ + step 3 ____cDDD + +needs exactly *8* OSDs, one for each chunk. If the hosts are in two +adjacent racks, the first four chunks can be placed in the first rack +and the last four in the second rack. So that recovering from the loss +of a single OSD does not require using bandwidth between the two +racks. + +For instance:: + + crush-steps='[ [ "choose", "rack", 2 ], [ "chooseleaf", "host", 4 ] ]' + +will create a ruleset that will select two crush buckets of type +*rack* and for each of them choose four OSDs, each of them located in +different buckets of type *host*. + +The ruleset can also be manually crafted for finer control. |