db-analyser: document pitfall of --repro-mempool-and-forge 2

Author: amesgen

ouroboros-consensus-cardano/README.md

@@ -166,6 +166,16 @@ Lastly the user can provide the analysis that should be run on the chain:
   - time spent in the mutator in microseconds
   - time spent in GC in microseconds
 
+  Currently, only NUM=1 or NUM=2 are supported. Note that with NUM=2, even for a
+  fully valid chain (like mainnet), you might get an error like this:
+
+  ```
+  Mempool rejected some of the on-chain txs: ... (OutsideValidityIntervalUTxO ...) ...
+  ```
+
+  Therefore, it is recommended to start with NUM=1, and only use NUM=2 when you
+  want to test the performance impact of a more filled mempool.
+
 * `--get-block-application-metrics NUM` computes different block application metrics every `NUM` blocks.
 It currently outputs block number, slot number, UTxO size in MB, and UTxO map size.
 In the [`scripts`](./scripts) directory we provide a `plot_utxo-growth.py` script that can be used to plot the these results.

ouroboros-consensus-cardano/src/unstable-cardano-tools/Cardano/Tools/DBAnalyser/Analysis.hs

@@ -816,7 +816,14 @@ reproMempoolForge numBlks env = do
               | rej@(Mempool.MempoolTxRejected tx _) <- results
               ]
         unless (null rejs) $ do
-          fail $ "Mempool rejected some of the on-chain txs: " <> show rejs
+          fail $ unlines $
+               ["Mempool rejected some of the on-chain txs: " <> show rejs]
+            <> case howManyBlocks of
+                 ReproMempoolForgeOneBlk -> []
+                 ReproMempoolForgeTwoBlks ->
+                   [ "This might be expected, see the db-analyser README."
+                   , "Consider trying again with `--repro-mempool-and-forge 1`."
+                   ]
 
       let scrutinee = case howManyBlocks of
             ReproMempoolForgeOneBlk  -> Just blk'