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ReStructuredText
416 lines
18 KiB
ReStructuredText
===========================
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FULL OSDMAP VERSION PRUNING
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===========================
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For each incremental osdmap epoch, the monitor will keep a full osdmap
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epoch in the store.
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While this is great when serving osdmap requests from clients, allowing
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us to fulfill their request without having to recompute the full osdmap
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from a myriad of incrementals, it can also become a burden once we start
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keeping an unbounded number of osdmaps.
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The monitors will attempt to keep a bounded number of osdmaps in the store.
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This number is defined (and configurable) via ``mon_min_osdmap_epochs``, and
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defaults to 500 epochs. Generally speaking, we will remove older osdmap
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epochs once we go over this limit.
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However, there are a few constraints to removing osdmaps. These are all
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defined in ``OSDMonitor::get_trim_to()``.
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In the event one of these conditions is not met, we may go over the bounds
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defined by ``mon_min_osdmap_epochs``. And if the cluster does not meet the
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trim criteria for some time (e.g., unclean pgs), the monitor may start
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keeping a lot of osdmaps. This can start putting pressure on the underlying
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key/value store, as well as on the available disk space.
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One way to mitigate this problem would be to stop keeping full osdmap
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epochs on disk. We would have to rebuild osdmaps on-demand, or grab them
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from cache if they had been recently served. We would still have to keep
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at least one osdmap, and apply all incrementals on top of either this
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oldest map epoch kept in the store or a more recent map grabbed from cache.
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While this would be feasible, it seems like a lot of cpu (and potentially
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IO) would be going into rebuilding osdmaps.
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Additionally, this would prevent the aforementioned problem going forward,
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but would do nothing for stores currently in a state that would truly
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benefit from not keeping osdmaps.
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This brings us to full osdmap pruning.
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Instead of not keeping full osdmap epochs, we are going to prune some of
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them when we have too many.
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Deciding whether we have too many will be dictated by a configurable option
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``mon_osdmap_full_prune_min`` (default: 10000). The pruning algorithm will be
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engaged once we go over this threshold.
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We will not remove all ``mon_osdmap_full_prune_min`` full osdmap epochs
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though. Instead, we are going to poke some holes in the sequence of full
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maps. By default, we will keep one full osdmap per 10 maps since the last
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map kept; i.e., if we keep epoch 1, we will also keep epoch 10 and remove
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full map epochs 2 to 9. The size of this interval is configurable with
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``mon_osdmap_full_prune_interval``.
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Essentially, we are proposing to keep ~10% of the full maps, but we will
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always honour the minimum number of osdmap epochs, as defined by
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``mon_min_osdmap_epochs``, and these won't be used for the count of the
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minimum versions to prune. For instance, if we have on-disk versions
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[1..50000], we would allow the pruning algorithm to operate only over
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osdmap epochs [1..49500); but, if have on-disk versions [1..10200], we
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won't be pruning because the algorithm would only operate on versions
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[1..9700), and this interval contains less versions than the minimum
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required by ``mon_osdmap_full_prune_min``.
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ALGORITHM
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=========
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Say we have 50,000 osdmap epochs in the store, and we're using the
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defaults for all configurable options.
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::
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-----------------------------------------------------------
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|1|2|..|10|11|..|100|..|1000|..|10000|10001|..|49999|50000|
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-----------------------------------------------------------
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^ first last ^
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We will prune when all the following constraints are met:
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1. number of versions is greater than ``mon_min_osdmap_epochs``;
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2. the number of versions between ``first`` and ``prune_to`` is greater (or
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equal) than ``mon_osdmap_full_prune_min``, with ``prune_to`` being equal to
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``last`` minus ``mon_min_osdmap_epochs``.
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If any of these conditions fails, we will *not* prune any maps.
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Furthermore, if it is known that we have been pruning, but since then we
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are no longer satisfying at least one of the above constraints, we will
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not continue to prune. In essence, we only prune full osdmaps if the
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number of epochs in the store so warrants it.
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As pruning will create gaps in the sequence of full maps, we need to keep
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track of the intervals of missing maps. We do this by keeping a manifest of
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pinned maps -- i.e., a list of maps that, by being pinned, are not to be
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pruned.
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While pinned maps are not removed from the store, maps between two consecutive
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pinned maps will; and the number of maps to be removed will be dictated by the
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configurable option ``mon_osdmap_full_prune_interval``. The algorithm makes an
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effort to keep pinned maps apart by as many maps as defined by this option,
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but in the event of corner cases it may allow smaller intervals. Additionally,
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as this is a configurable option that is read any time a prune iteration
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occurs, there is the possibility this interval will change if the user changes
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this config option.
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Pinning maps is performed lazily: we will be pinning maps as we are removing
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maps. This grants us more flexibility to change the prune interval while
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pruning is happening, but also simplifies considerably the algorithm, as well
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as the information we need to keep in the manifest. Below we show a simplified
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version of the algorithm:::
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manifest.pin(first)
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last_to_prune = last - mon_min_osdmap_epochs
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while manifest.get_last_pinned() + prune_interval < last_to_prune AND
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last_to_prune - first > mon_min_osdmap_epochs AND
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last_to_prune - first > mon_osdmap_full_prune_min AND
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num_pruned < mon_osdmap_full_prune_txsize:
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last_pinned = manifest.get_last_pinned()
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new_pinned = last_pinned + prune_interval
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manifest.pin(new_pinned)
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for e in (last_pinned .. new_pinned):
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store.erase(e)
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++num_pruned
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In essence, the algorithm ensures that the first version in the store is
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*always* pinned. After all, we need a starting point when rebuilding maps, and
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we can't simply remove the earliest map we have; otherwise we would be unable
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to rebuild maps for the very first pruned interval.
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Once we have at least one pinned map, each iteration of the algorithm can
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simply base itself on the manifest's last pinned map (which we can obtain by
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reading the element at the tail of the manifest's pinned maps list).
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We'll next need to determine the interval of maps to be removed: all the maps
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from ``last_pinned`` up to ``new_pinned``, which in turn is nothing more than
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``last_pinned`` plus ``mon_osdmap_full_prune_interval``. We know that all maps
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between these two values, ``last_pinned`` and ``new_pinned`` can be removed,
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considering ``new_pinned`` has been pinned.
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The algorithm ceases to execute as soon as one of the two initial
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preconditions is not met, or if we do not meet two additional conditions that
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have no weight on the algorithm's correctness:
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1. We will stop if we are not able to create a new pruning interval properly
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aligned with ``mon_osdmap_full_prune_interval`` that is lower than
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``last_pruned``. There is no particular technical reason why we enforce
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this requirement, besides allowing us to keep the intervals with an
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expected size, and preventing small, irregular intervals that would be
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bound to happen eventually (e.g., pruning continues over the course of
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several iterations, removing one or two or three maps each time).
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2. We will stop once we know that we have pruned more than a certain number of
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maps. This value is defined by ``mon_osdmap_full_prune_txsize``, and
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ensures we don't spend an unbounded number of cycles pruning maps. We don't
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enforce this value religiously (deletes do not cost much), but we make an
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effort to honor it.
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We could do the removal in one go, but we have no idea how long that would
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take. Therefore, we will perform several iterations, removing at most
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``mon_osdmap_full_prune_txsize`` osdmaps per iteration.
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In the end, our on-disk map sequence will look similar to::
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------------------------------------------
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|1|10|20|30|..|49500|49501|..|49999|50000|
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------------------------------------------
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^ first last ^
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Because we are not pruning all versions in one go, we need to keep state
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about how far along on our pruning we are. With that in mind, we have
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created a data structure, ``osdmap_manifest_t``, that holds the set of pinned
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maps:::
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struct osdmap_manifest_t:
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set<version_t> pinned;
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Given we are only pinning maps while we are pruning, we don't need to keep
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track of additional state about the last pruned version. We know as a matter
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of fact that we have pruned all the intermediate maps between any two
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consecutive pinned maps.
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The question one could ask, though, is how can we be sure we pruned all the
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intermediate maps if, for instance, the monitor crashes. To ensure we are
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protected against such an event, we always write the osdmap manifest to disk
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on the same transaction that is deleting the maps. This way we have the
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guarantee that, if the monitor crashes, we will read the latest version of the
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manifest: either containing the newly pinned maps, meaning we also pruned the
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in-between maps; or we will find the previous version of the osdmap manifest,
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which will not contain the maps we were pinning at the time we crashed, given
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the transaction on which we would be writing the updated osdmap manifest was
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not applied (alongside with the maps removal).
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The osdmap manifest will be written to the store each time we prune, with an
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updated list of pinned maps. It is written in the transaction effectively
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pruning the maps, so we guarantee the manifest is always up to date. As a
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consequence of this criteria, the first time we will write the osdmap manifest
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is the first time we prune. If an osdmap manifest does not exist, we can be
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certain we do not hold pruned map intervals.
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We will rely on the manifest to ascertain whether we have pruned maps
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intervals. In theory, this will always be the on-disk osdmap manifest, but we
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make sure to read the on-disk osdmap manifest each time we update from paxos;
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this way we always ensure having an up to date in-memory osdmap manifest.
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Once we finish pruning maps, we will keep the manifest in the store, to
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allow us to easily find which maps have been pinned (instead of checking
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the store until we find a map). This has the added benefit of allowing us to
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quickly figure out which is the next interval we need to prune (i.e., last
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pinned plus the prune interval). This doesn't however mean we will forever
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keep the osdmap manifest: the osdmap manifest will no longer be required once
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the monitor trims osdmaps and the earliest available epoch in the store is
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greater than the last map we pruned.
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The same conditions from ``OSDMonitor::get_trim_to()`` that force the monitor
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to keep a lot of osdmaps, thus requiring us to prune, may eventually change
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and allow the monitor to remove some of its oldest maps.
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MAP TRIMMING
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------------
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If the monitor trims maps, we must then adjust the osdmap manifest to
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reflect our pruning status, or remove the manifest entirely if it no longer
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makes sense to keep it. For instance, take the map sequence from before, but
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let us assume we did not finish pruning all the maps.::
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-------------------------------------------------------------
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|1|10|20|30|..|490|500|501|502|..|49500|49501|..|49999|50000|
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-------------------------------------------------------------
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^ first ^ pinned.last() last ^
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pinned = {1, 10, 20, ..., 490, 500}
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Now let us assume that the monitor will trim up to epoch 501. This means
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removing all maps prior to epoch 501, and updating the ``first_committed``
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pointer to ``501``. Given removing all those maps would invalidate our
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existing pruning efforts, we can consider our pruning has finished and drop
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our osdmap manifest. Doing so also simplifies starting a new prune, if all
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the starting conditions are met once we refreshed our state from the
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store.
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We would then have the following map sequence: ::
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---------------------------------------
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|501|502|..|49500|49501|..|49999|50000|
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---------------------------------------
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^ first last ^
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However, imagine a slightly more convoluted scenario: the monitor will trim
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up to epoch 491. In this case, epoch 491 has been previously pruned from the
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store.
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Given we will always need to have the oldest known map in the store, before
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we trim we will have to check whether that map is in the prune interval
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(i.e., if said map epoch belongs to ``[ pinned.first()..pinned.last() )``).
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If so, we need to check if this is a pinned map, in which case we don't have
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much to be concerned aside from removing lower epochs from the manifest's
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pinned list. On the other hand, if the map being trimmed to is not a pinned
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map, we will need to rebuild said map and pin it, and only then will we remove
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the pinned maps prior to the map's epoch.
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In this case, we would end up with the following sequence:::
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-----------------------------------------------
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|491|500|501|502|..|49500|49501|..|49999|50000|
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-----------------------------------------------
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^ ^- pinned.last() last ^
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`- first
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There is still an edge case that we should mention. Consider that we are
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going to trim up to epoch 499, which is the very last pruned epoch.
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Much like the scenario above, we would end up writing osdmap epoch 499 to
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the store; but what should we do about pinned maps and pruning?
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The simplest solution is to drop the osdmap manifest. After all, given we
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are trimming to the last pruned map, and we are rebuilding this map, we can
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guarantee that all maps greater than e 499 are sequential (because we have
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not pruned any of them). In essence, dropping the osdmap manifest in this
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case is essentially the same as if we were trimming over the last pruned
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epoch: we can prune again later if we meet the required conditions.
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And, with this, we have fully dwelled into full osdmap pruning. Later in this
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document one can find detailed `REQUIREMENTS, CONDITIONS & INVARIANTS` for the
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whole algorithm, from pruning to trimming. Additionally, the next section
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details several additional checks to guarantee the sanity of our configuration
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options. Enjoy.
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CONFIGURATION OPTIONS SANITY CHECKS
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-----------------------------------
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We perform additional checks before pruning to ensure all configuration
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options involved are sane:
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1. If ``mon_osdmap_full_prune_interval`` is zero we will not prune; we
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require an actual positive number, greater than one, to be able to prune
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maps. If the interval is one, we would not actually be pruning any maps, as
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the interval between pinned maps would essentially be a single epoch. This
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means we would have zero maps in-between pinned maps, hence no maps would
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ever be pruned.
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2. If ``mon_osdmap_full_prune_min`` is zero we will not prune; we require a
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positive, greater than zero, value so we know the threshold over which we
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should prune. We don't want to guess.
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3. If ``mon_osdmap_full_prune_interval`` is greater than
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``mon_osdmap_full_prune_min`` we will not prune, as it is impossible to
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ascertain a proper prune interval.
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4. If ``mon_osdmap_full_prune_txsize`` is lower than
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``mon_osdmap_full_prune_interval`` we will not prune; we require a
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``txsize`` with a value at least equal than ``interval``, and (depending on
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the value of the latter) ideally higher.
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REQUIREMENTS, CONDITIONS & INVARIANTS
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-------------------------------------
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REQUIREMENTS
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~~~~~~~~~~~~
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* All monitors in the quorum need to support pruning.
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* Once pruning has been enabled, monitors not supporting pruning will not be
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allowed in the quorum, nor will be allowed to synchronize.
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* Removing the osdmap manifest results in disabling the pruning feature quorum
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requirement. This means that monitors not supporting pruning will be allowed
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to synchronize and join the quorum, granted they support any other features
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required.
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CONDITIONS & INVARIANTS
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~~~~~~~~~~~~~~~~~~~~~~~
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* Pruning has never happened, or we have trimmed past its previous
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intervals:::
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invariant: first_committed > 1
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condition: pinned.empty() AND !store.exists(manifest)
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* Pruning has happened at least once:::
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invariant: first_committed > 0
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invariant: !pinned.empty())
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invariant: pinned.first() == first_committed
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invariant: pinned.last() < last_committed
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precond: pinned.last() < prune_to AND
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pinned.last() + prune_interval < prune_to
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postcond: pinned.size() > old_pinned.size() AND
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(for each v in [pinned.first()..pinned.last()]:
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if pinned.count(v) > 0: store.exists_full(v)
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else: !store.exists_full(v)
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)
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* Pruning has finished:::
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invariant: first_committed > 0
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invariant: !pinned.empty()
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invariant: pinned.first() == first_committed
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invariant: pinned.last() < last_committed
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condition: pinned.last() == prune_to OR
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pinned.last() + prune_interval < prune_to
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* Pruning intervals can be trimmed:::
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precond: OSDMonitor::get_trim_to() > 0
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condition: !pinned.empty()
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invariant: pinned.first() == first_committed
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invariant: pinned.last() < last_committed
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invariant: pinned.first() <= OSDMonitor::get_trim_to()
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invariant: pinned.last() >= OSDMonitor::get_trim_to()
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* Trim pruned intervals:::
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invariant: !pinned.empty()
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invariant: pinned.first() == first_committed
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invariant: pinned.last() < last_committed
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invariant: pinned.first() <= OSDMonitor::get_trim_to()
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invariant: pinned.last() >= OSDMonitor::get_trim_to()
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postcond: pinned.empty() OR
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(pinned.first() == OSDMonitor::get_trim_to() AND
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pinned.last() > pinned.first() AND
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(for each v in [0..pinned.first()]:
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!store.exists(v) AND
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!store.exists_full(v)
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) AND
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(for each m in [pinned.first()..pinned.last()]:
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if pinned.count(m) > 0: store.exists_full(m)
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else: !store.exists_full(m) AND store.exists(m)
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)
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)
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postcond: !pinned.empty() OR
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(!store.exists(manifest) AND
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(for each v in [pinned.first()..pinned.last()]:
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!store.exists(v) AND
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!store.exists_full(v)
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)
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)
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