WT-2764 Replace write leaves phase of checkpoints. (#2876)

Instead, get multi-threaded writes by dialling down the eviction dirty trigger.

Change eviction_dirty_* defaults (to 20/5).

Make sure all files are available for eviction before starting fsync.

Take more care excluding clean files from checkpoints.

Only use scrubbing mode when there is minimal cache pressure.

Avoid penalizing read-only operations with dirty eviction targets.

Enable scrubbing when eviction is keeping space available in cache.

Add stats for checkpoint scrubbing phase.

Improve cache scrubbing with big caches and slow I/O.

Use the rate of bytes written from cache to decide how long to wait for
the dirty bytes to come down.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/docs/upgrading.dox b/src/docs/upgrading.dox
index db40f5c2b55..9d3d2239bb4 100644
--- a/src/docs/upgrading.dox
+++ b/src/docs/upgrading.dox
@@ -2,6 +2,20 @@
 
 @section version_281 Upgrading to Version 2.8.1
 <dl>
+<dt>Cache management defaults</dt>
+<dd>
+The default values for the \c eviction_dirty_target and \c
+eviction_dirty_trigger settings to ::wiredtiger_open have changed to 5 and 20,
+respectively.  This means that by default, WiredTiger will start writing dirty
+pages from cache when it becomes 5% dirty and will throttle activity to keep
+the volume of dirty data in cache under 20%.  For write-heavy workloads, the
+new defaults may result in lower throughput and more threads writing to data
+files concurrently.
+
+These settings also now determine how much work is done at the beginning of a
+checkpoint to make the critical section of checkpoints complete more quickly.
+</dd>
+
 <dt>Checkpoint server created checkpoint names</dt>
 <dd>
 The ::wiredtiger_open checkpoint configuration no longer supports the