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crush: document tunables and rule step set_
Signed-off-by: Loic Dachary <loic@dachary.org>
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Loic Dachary
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0c2f5f7214
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42aa85bd95
@ -76,6 +76,9 @@ extern struct crush_rule *crush_make_rule(int len, int ruleset, int type, int mi
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*
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* - __CRUSH_RULE_NOOP__ do nothing.
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* - __CRUSH_RULE_TAKE__ select the __arg1__ item
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* - __CRUSH_RULE_EMIT__ append the selection to the results and clear
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* the selection
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*
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* - __CRUSH_RULE_CHOOSE_FIRSTN__ and __CRUSH_RULE_CHOOSE_INDEP__
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* recursively explore each bucket currently selected, looking for
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* __arg1__ items of type __arg2__ and select them.
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@ -83,17 +86,54 @@ extern struct crush_rule *crush_make_rule(int len, int ruleset, int type, int mi
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* recursively explore each bucket currently selected, looking for
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* __arg1__ leaves within all the buckets of type __arg2__ and
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* select them.
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* - __CRUSH_RULE_EMIT__ append the selection to the results and clear
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* the selection
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*
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* In all __CHOOSE__ steps, if __arg1__ is zero, the number of items
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* to select is determined by the __max_result__ argument of
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* crush_do_rule(), i.e. __arg1__ is __max_result__ minus the number of
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* items already in the result.
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*
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* - __CRUSH_RULE_SET_CHOOSE_TRIES__ and __CRUSH_RULE_SET_CHOOSELEAF_TRIES__
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*
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* The CHOOSE_FIRSTN and CHOOSE_INDEP rule step look for buckets of
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* a given type, randomly selecting them. If they are unlucky and
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* find the same bucket twice, they will try N+1 times (N being the
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* value of the choose_total_tries tunable). If there is a previous
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* SET_CHOOSE_TRIES step in the same rule, it will try C times
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* instead (C being the value of the argument of the
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* SET_CHOOSE_TRIES step).
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*
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* Note: the __choose_total_tries__ tunable defined in crush_map is
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* the number of retry, not the number of tries. The number of tries
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* is the number of retry+1. The SET_CHOOSE_TRIES rule step sets the
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* number of tries and does not need the + 1. This confusing
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* difference is inherited from an off-by-one bug from years ago.
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*
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* The CHOOSELEAF_FIRSTN and CHOOSELEAF_INDEP rule step do the same
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* as CHOOSE_FIRSTN and CHOOSE_INDEP but also recursively explore
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* each bucket found, looking for a single device. The same device
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* may be found in two different buckets because the crush map is
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* not a strict hierarchy, it is a DAG. When such a collision
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* happens, they will try again. The number of times they try to
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* find a non colliding device is:
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*
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* - If FIRSTN and there is no previous SET_CHOOSELEAF_TRIES rule
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* step: try N + 1 times (N being the value of the
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* __choose_total_tries__ tunable defined in crush_map)
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*
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* - If FIRSTN and there is a previous SET_CHOOSELEAF_TRIES rule
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* step: try P times (P being the value of the argument of the
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* SET_CHOOSELEAF_TRIES rule step)
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*
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* - If INDEP and there is no previous SET_CHOOSELEAF_TRIES rule
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* step: try 1 time.
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*
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* - If INDEP and there is a previous SET_CHOOSELEAF_TRIES rule step: try
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* P times (P being the value of the argument of the SET_CHOOSELEAF_TRIES
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* rule step)
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*
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* @param rule the rule in which the step is inserted
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* @param pos the zero based step index
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* @param op one of __CRUSH_RULE_NOOP__, __CRUSH_RULE_TAKE__, __CRUSH_RULE_CHOOSE_FIRSTN__, __CRUSH_RULE_CHOOSE_INDEP__, __CRUSH_RULE_CHOOSELEAF_FIRSTN__, __CRUSH_RULE_CHOOSELEAF_INDEP__ or __CRUSH_RULE_EMIT__
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* @param op one of __CRUSH_RULE_NOOP__, __CRUSH_RULE_TAKE__, __CRUSH_RULE_CHOOSE_FIRSTN__, __CRUSH_RULE_CHOOSE_INDEP__, __CRUSH_RULE_CHOOSELEAF_FIRSTN__, __CRUSH_RULE_CHOOSELEAF_INDEP__, __CRUSH_RULE_SET_CHOOSE_TRIES__, __CRUSH_RULE_SET_CHOOSELEAF_TRIES__ or __CRUSH_RULE_EMIT__
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* @param arg1 first argument for __op__
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* @param arg2 second argument for __op__
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*/
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@ -315,28 +315,54 @@ struct crush_map {
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*/
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__s32 max_devices;
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/*! choose local retries before re-descent */
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/*! Backward compatibility tunable. It implements a bad solution
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* and must always be set to 0 except for backward compatibility
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* purposes
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*/
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__u32 choose_local_tries;
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/*! choose local attempts using a fallback permutation before
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*! re-descent */
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/*! Backward compatibility tunable. It implements a bad solution
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* and must always be set to 0 except for backward compatibility
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* purposes
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*/
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__u32 choose_local_fallback_tries;
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/*! choose attempts before giving up */
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/*! Tunable. The default value when the CHOOSE_TRIES or
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* CHOOSELEAF_TRIES steps are omitted in a rule. See the
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* documentation for crush_rule_set_step() for more
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* information
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*/
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__u32 choose_total_tries;
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/*! attempt chooseleaf inner descent once for firstn mode; on
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*! reject retry outer descent. Note that this does *not*
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*! apply to a collision: in that case we will retry as we used
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*! to. */
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/*! Backward compatibility tunable. It should always be set
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* to 1 except for backward compatibility. Implemented in 2012
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* it was generalized late 2013 and is mostly unused except
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* in one border case, reason why it must be set to 1.
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*
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* Attempt chooseleaf inner descent once for firstn mode; on
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* reject retry outer descent. Note that this does *not*
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* apply to a collision: in that case we will retry as we
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* used to.
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*/
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__u32 chooseleaf_descend_once;
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/*! if non-zero, feed r into chooseleaf, bit-shifted right by (r-1)
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*! bits. a value of 1 is best for new clusters. for legacy clusters
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*! that want to limit reshuffling, a value of 3 or 4 will make the
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*! mappings line up a bit better with previous mappings. */
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/*! Backward compatibility tunable. It is a fix for bad
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* mappings implemented in 2014 at
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* https://github.com/ceph/ceph/pull/1185. It should always
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* be set to 1 except for backward compatibility.
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*
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* If non-zero, feed r into chooseleaf, bit-shifted right by
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* (r-1) bits. a value of 1 is best for new clusters. for
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* legacy clusters that want to limit reshuffling, a value of
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* 3 or 4 will make the mappings line up a bit better with
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* previous mappings.
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*/
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__u8 chooseleaf_vary_r;
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/*! if true, it makes chooseleaf firstn to return stable results (if
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*! no local retry) so that data migrations would be optimal when some
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*! device fails. */
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/*! Backward compatibility tunable. It is an improvement that
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* avoids unnecessary mapping changes, implemented at
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* https://github.com/ceph/ceph/pull/6572 and explained in
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* this post: "chooseleaf may cause some unnecessary pg
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* migrations" in October 2015
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* https://www.mail-archive.com/ceph-devel@vger.kernel.org/msg26075.html
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* It should always be set to 1 except for backward compatibility.
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*/
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__u8 chooseleaf_stable;
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/*! @cond INTERNAL */
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@ -354,12 +380,17 @@ struct crush_map {
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size_t working_size;
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#ifndef __KERNEL__
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/*
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* version 0 (original) of straw_calc has various flaws. version 1
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* fixes a few of them.
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/*! @endcond */
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/*! Backward compatibility tunable. It is a fix for the straw
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* scaler values for the straw algorithm which is deprecated
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* (straw2 replaces it) implemented at
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* https://github.com/ceph/ceph/pull/3057. It should always
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* be set to 1 except for backward compatibility.
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*
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*/
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__u8 straw_calc_version;
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/*! @cond INTERNAL */
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/*
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* allowed bucket algs is a bitmask, here the bit positions
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* are CRUSH_BUCKET_*. note that these are *bits* and
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