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-=============
-OSD Throttles
-=============
-
-There are three significant throttles in the filestore: wbthrottle,
-op_queue_throttle, and a throttle based on journal usage.
-
-WBThrottle
-----------
-The WBThrottle is defined in src/os/filestore/WBThrottle.[h,cc] and
-included in FileStore as FileStore::wbthrottle.  The intention is to
-bound the amount of outstanding IO we need to do to flush the journal.
-At the same time, we don't want to necessarily do it inline in case we
-might be able to combine several IOs on the same object close together
-in time.  Thus, in FileStore::_write, we queue the fd for asyncronous
-flushing and block in FileStore::_do_op if we have exceeded any hard
-limits until the background flusher catches up.
-
-The relevant config options are filestore_wbthrottle*.  There are
-different defaults for xfs and btrfs.  Each set has hard and soft
-limits on bytes (total dirty bytes), ios (total dirty ios), and
-inodes (total dirty fds).  The WBThrottle will begin flushing
-when any of these hits the soft limit and will block in throttle()
-while any has exceeded the hard limit.
-
-Tighter soft limits will cause writeback to happen more quickly,
-but may cause the OSD to miss oportunities for write coalescing.
-Tighter hard limits may cause a reduction in latency variance by
-reducing time spent flushing the journal, but may reduce writeback
-parallelism.
-
-op_queue_throttle
------------------
-The op queue throttle is intended to bound the amount of queued but
-uncompleted work in the filestore by delaying threads calling
-queue_transactions more and more based on how many ops and bytes are
-currently queued.  The throttle is taken in queue_transactions and
-released when the op is applied to the filesystem.  This period
-includes time spent in the journal queue, time spent writing to the
-journal, time spent in the actual op queue, time spent waiting for the
-wbthrottle to open up (thus, the wbthrottle can push back indirectly
-on the queue_transactions caller), and time spent actually applying
-the op to the filesystem.  A BackoffThrottle is used to gradually
-delay the queueing thread after each throttle becomes more than
-filestore_queue_low_threshhold full (a ratio of
-filestore_queue_max_(bytes|ops)).  The throttles will block once the
-max value is reached (filestore_queue_max_(bytes|ops)).
-
-The significant config options are:
-filestore_queue_low_threshhold
-filestore_queue_high_threshhold
-filestore_expected_throughput_ops
-filestore_expected_throughput_bytes
-filestore_queue_high_delay_multiple
-filestore_queue_max_delay_multiple
-
-While each throttle is at less than low_threshhold of the max,
-no delay happens.  Between low and high, the throttle will
-inject a per-op delay (per op or byte) ramping from 0 at low to
-high_delay_multiple/expected_throughput at high.  From high to
-1, the delay will ramp from high_delay_multiple/expected_throughput
-to max_delay_multiple/expected_throughput.
-
-filestore_queue_high_delay_multiple and
-filestore_queue_max_delay_multiple probably do not need to be
-changed.
-
-Setting these properly should help to smooth out op latencies by
-mostly avoiding the hard limit.
-
-See FileStore::throttle_ops and FileSTore::thottle_bytes.
-
-journal usage throttle
-----------------------
-See src/os/filestore/JournalThrottle.h/cc
-
-The intention of the journal usage throttle is to gradually slow
-down queue_transactions callers as the journal fills up in order
-to smooth out hiccup during filestore syncs.  JournalThrottle
-wraps a BackoffThrottle and tracks journaled but not flushed
-journal entries so that the throttle can be released when the
-journal is flushed.  The configs work very similarly to the
-op_queue_throttle.
-
-The significant config options are:
-journal_throttle_low_threshhold
-journal_throttle_high_threshhold
-filestore_expected_throughput_ops
-filestore_expected_throughput_bytes
-journal_throttle_high_multiple
-journal_throttle_max_multiple
-
-.. literalinclude:: osd_throttles.txt