During IBD, when doing pruning, prune 10% extra to avoid pruning again soon after

Pruning forces a chainstate flush, which can defeat the dbcache and harm performance significantly.

Author: luke-jr

src/validation.cpp

@@ -3391,6 +3391,15 @@ void FindFilesToPrune(std::set<int>& setFilesToPrune, uint64_t nPruneAfterHeight
     int count=0;
 
     if (nCurrentUsage + nBuffer >= nPruneTarget) {
+        // On a prune event, the chainstate DB is flushed.
+        // To avoid excessive prune events negating the benefit of high dbcache
+        // values, we should not prune too rapidly.
+        // So when pruning in IBD, increase the buffer a bit to avoid a re-prune too soon.
+        if (IsInitialBlockDownload()) {
+            // Since this is only relevant during IBD, we use a fixed 10%
+            nBuffer += nPruneTarget / 10;
+        }
+
         for (int fileNumber = 0; fileNumber < nLastBlockFile; fileNumber++) {
             nBytesToPrune = vinfoBlockFile[fileNumber].nSize + vinfoBlockFile[fileNumber].nUndoSize;