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29 lines
1.2 KiB
ReStructuredText
29 lines
1.2 KiB
ReStructuredText
==================
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Writeback Throttle
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==================
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Previously, the filestore had a problem when handling large numbers of
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small ios. We throttle dirty data implicitely via the journal, but
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a large number of inodes can be dirtied without filling the journal
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resulting in a very long sync time when the sync finally does happen.
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The flusher was not an adequate solution to this problem since it
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forced writeback of small writes too eagerly killing performance.
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WBThrottle tracks unflushed io per hobject_t and ::fsyncs in lru
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order once the start_flusher threshhold is exceeded for any of
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dirty bytes, dirty ios, or dirty inodes. While any of these exceed
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the hard_limit, we block on throttle() in _do_op.
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See src/os/WBThrottle.h, src/osd/WBThrottle.cc
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To track the open FDs through the writeback process, there is now an
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fdcache to cache open fds. lfn_open now returns a cached FDRef which
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implicitely closes the fd once all references have expired.
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Filestore syncs have a sideeffect of flushing all outstanding objects
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in the wbthrottle.
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lfn_unlink clears the cached FDRef and wbthrottle entries for the
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unlinked object when then last link is removed and asserts that all
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outstanding FDRefs for that object are dead.
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