Make threshold for flushing more conservative.

Always leave a reasonable buffer of 50MB for usage from newly connected block (once over 50%) and increase the high water mark buffer to 200MB.

src/txdb.h

@@ -23,8 +23,10 @@ class uint256;
 
 //! Compensate for extra memory peak (x1.5-x1.9) at flush time.
 static constexpr int DB_PEAK_USAGE_FACTOR = 2;
-//! No need to flush if at least this much space still available.
+//! No need to periodic flush if at least this much space still available.
-static constexpr int MAX_BLOCK_COINSDB_USAGE = 100 * DB_PEAK_USAGE_FACTOR;
+static constexpr int MAX_BLOCK_COINSDB_USAGE = 200 * DB_PEAK_USAGE_FACTOR;
+//! Always periodic flush if less than this much space still available.
+static constexpr int MIN_BLOCK_COINSDB_USAGE = 50 * DB_PEAK_USAGE_FACTOR;
 //! -dbcache default (MiB)
 static const int64_t nDefaultDbCache = 450;
 //! max. -dbcache (MiB)

src/validation.cpp

@@ -2007,8 +2007,9 @@ bool static FlushStateToDisk(CValidationState &state, FlushStateMode mode, int n
     int64_t nMempoolSizeMax = GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000;
     int64_t cacheSize = pcoinsTip->DynamicMemoryUsage() * DB_PEAK_USAGE_FACTOR;
     int64_t nTotalSpace = nCoinCacheUsage + std::max<int64_t>(nMempoolSizeMax - nMempoolUsage, 0);
-    // The cache is large and we're within 10% and 100 MiB of the limit, but we have time now (not in the middle of a block processing).
+    // The cache is large and we're within 10% and 200 MiB or 50% and 50MiB of the limit, but we have time now (not in the middle of a block processing).
-    bool fCacheLarge = mode == FLUSH_STATE_PERIODIC && cacheSize > std::max((9 * nTotalSpace) / 10, nTotalSpace - MAX_BLOCK_COINSDB_USAGE * 1024 * 1024);
+    bool fCacheLarge = mode == FLUSH_STATE_PERIODIC && cacheSize > std::min(std::max(nTotalSpace / 2, nTotalSpace - MIN_BLOCK_COINSDB_USAGE * 1024 * 1024),
+                                                                            std::max((9 * nTotalSpace) / 10, nTotalSpace - MAX_BLOCK_COINSDB_USAGE * 1024 * 1024));
     // The cache is over the limit, we have to write now.
     bool fCacheCritical = mode == FLUSH_STATE_IF_NEEDED && cacheSize > nTotalSpace;
     // It's been a while since we wrote the block index to disk. Do this frequently, so we don't need to redownload after a crash.