Merge bitcoin#11658: During IBD, when doing pruning, prune 10% extra …

…to avoid pruning again soon after

ac51a26 During IBD, when doing pruning, prune 10% extra to avoid pruning again soon after (Luke Dashjr)

Pull request description:

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

  Alternative to bitcoin#11359

Tree-SHA512: 631e4e8f94f5699e98a2eff07204aa2b3b2325b2d92e8236b8c8d6a6730737a346e0ad86024e705f5a665b25e873ab0970ce7396740328a437c060f99e9ba4d9

src/validation.cpp

@@ -3974,6 +3974,15 @@ static void FindFilesToPrune(std::set<int>& setFilesToPrune, uint64_t nPruneAfte
     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;