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