SERVER-31767 Provide a window of snapshot history that is accessible for PIT reads

1 files changed, 77 insertions, 0 deletions

diff --git a/src/mongo/db/snapshot_window_util.h b/src/mongo/db/snapshot_window_util.h
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+/**
+ *    Copyright (C) 2018 MongoDB Inc.
+ *
+ *    This program is free software: you can redistribute it and/or  modify
+ *    it under the terms of the GNU Affero General Public License, version 3,
+ *    as published by the Free Software Foundation.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    GNU Affero General Public License for more details.
+ *
+ *    You should have received a copy of the GNU Affero General Public License
+ *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the GNU Affero General Public License in all respects
+ *    for all of the code used other than as permitted herein. If you modify
+ *    file(s) with this exception, you may extend this exception to your
+ *    version of the file(s), but you are not obligated to do so. If you do not
+ *    wish to do so, delete this exception statement from your version. If you
+ *    delete this exception statement from all source files in the program,
+ *    then also delete it in the license file.
+ */
+
+#pragma once
+
+namespace mongo {
+
+class OperationContext;
+
+/**
+ * Function helpers to manipulate parameters affecting the snapshot time window size that the
+ * storage engine maintains of available snapshots for point-in-time reads.
+ */
+namespace SnapshotWindowUtil {
+
+/**
+ * Attempts to increase the setting that controls the window of time between stable_timestamp and
+ * oldest_timestamp, in order to provide a greater range of available snapshots for point-in-time
+ * operations. The window will not be increased, however, if the cache pressure is currently too
+ * high. This function will be called when server requests return SnapshotTooOld (or similar)
+ * errors. Note that this will not immediately affect the oldest_timestamp. Rather, it affects
+ * actions taken next time oldest_timestamp is updated, usually when the stable timestamp is
+ * advanced.
+ *
+ * Implements an additive increase algorithm.
+ *
+ * Calling many times all at once has the same effect as calling once. The last update time is
+ * tracked and attempts to increase the window are limited to once in
+ * minMillisBetweenSnapshotWindowInc. This is to protect against a sudden wave of function calls due
+ * to simultaneous SnapshotTooOld errors. Some time must be allowed for the increased target
+ * snapshot window size to have an effect. The target size can also never exceed
+ * maxTargetSnapshotHistoryWindowInSeconds.
+ */
+void increaseTargetSnapshotWindowSize(OperationContext* opCtx);
+
+/**
+ * Attempts to decrease (if not already zero) the setting that affects the size of the window of
+ * time between stable_timestamp and oldest_timestamp in order to reduce storage engine cache
+ * pressure. The window target will not be decreased, however, if the cache is not currently under
+ * pressure. Pressure can occur when too much history is being maintained for point-in-time
+ * snapshots. Note that this will not necessarily immediately affect the actual window size; rather,
+ * it affects actions taken whenever oldest_timestamp is updated, usually when the stable timestamp
+ * is advanced.
+ *
+ * This will make one attempt to immediately adjust the window size if possible.
+ *
+ * Implements a multiplicative decrease algorithm.
+ */
+void decreaseTargetSnapshotWindowSize(OperationContext* opCtx);
+
+}  // namespace SnapshotWindowUtil
+}  // namespace mongo