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83 lines
3.8 KiB
Go
83 lines
3.8 KiB
Go
// Copyright 2023 The LevelDB-Go and Pebble Authors. All rights reserved. Use
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// of this source code is governed by a BSD-style license that can be found in
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// the LICENSE file.
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package record
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// RotationHelper is a type used to inform the decision of rotating a record log
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// file.
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//
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// The assumption is that multiple records can be coalesced into a single record
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// (called a snapshot). Starting a new file, where the first record is a
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// snapshot of the current state is referred to as "rotating" the log.
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//
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// Normally we rotate files when a certain file size is reached. But in certain
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// cases (e.g. contents become very large), this can result in too frequent
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// rotation. This helper contains logic to impose extra conditions on the
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// rotation.
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//
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// The rotation helper uses "size" as a unit-less estimation that is correlated
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// with the on-disk size of a record or snapshot.
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type RotationHelper struct {
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// lastSnapshotSize is the size of the last snapshot.
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lastSnapshotSize int64
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// sizeSinceLastSnapshot is the sum of sizes of records applied since the last
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// snapshot.
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sizeSinceLastSnapshot int64
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lastRecordSize int64
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}
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// AddRecord makes the rotation helper aware of a new record.
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func (rh *RotationHelper) AddRecord(recordSize int64) {
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rh.sizeSinceLastSnapshot += recordSize
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rh.lastRecordSize = recordSize
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}
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// ShouldRotate returns whether we should start a new log file (with a snapshot).
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// Does not need to be called if other rotation factors (log file size) are not
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// satisfied.
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func (rh *RotationHelper) ShouldRotate(nextSnapshotSize int64) bool {
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// The primary goal is to ensure that when reopening a log file, the number of
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// edits that need to be replayed on top of the snapshot is "sane" while
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// keeping the rotation frequency as low as possible.
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//
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// For the purposes of this description, we assume that the log is mainly
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// storing a collection of "entries", with edits adding or removing entries.
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// Consider the following cases:
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//
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// - The number of live entries is roughly stable: after writing the snapshot
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// (with S entries), we require that there be enough edits such that the
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// cumulative number of entries in those edits, E, be greater than S. This
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// will ensure that at most 50% of data written out is due to rotation.
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//
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// - The number of live entries K in the DB is shrinking drastically, say from
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// S to S/10: After this shrinking, E = 0.9S, and so if we used the previous
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// snapshot entry count, S, as the threshold that needs to be exceeded, we
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// will further delay the snapshot writing. Which means on reopen we will
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// need to replay 0.9S edits to get to a version with 0.1S entries. It would
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// be better to create a new snapshot when E exceeds the number of entries in
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// the current version.
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//
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// - The number of live entries L in the DB is growing; say the last snapshot
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// had S entries, and now we have 10S entries, so E = 9S. If we required
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// that E is at least the current number of entries, we would further delay
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// writing a new snapshot (which is not desirable).
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//
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// The logic below uses the min of the last snapshot size count and the size
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// count in the current version.
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return rh.sizeSinceLastSnapshot > rh.lastSnapshotSize || rh.sizeSinceLastSnapshot > nextSnapshotSize
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}
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// Rotate makes the rotation helper aware that we are rotating to a new snapshot
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// (to which we will apply the latest edit).
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func (rh *RotationHelper) Rotate(snapshotSize int64) {
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rh.lastSnapshotSize = snapshotSize
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rh.sizeSinceLastSnapshot = rh.lastRecordSize
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}
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// DebugInfo returns the last snapshot size and size of the edits since the last
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// snapshot; used for testing and debugging.
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func (rh *RotationHelper) DebugInfo() (lastSnapshotSize int64, sizeSinceLastSnapshot int64) {
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return rh.lastSnapshotSize, rh.sizeSinceLastSnapshot
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}
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