A bit late to the party, but I just tried #12653 and noticed some non-intuitive behavior (on macOS), that's probably worth explaining release notes and help.

Before this new feature I kept the entire datadir on an SSD drive, so I launch with: `bitcoind -datadir=/Volumes/SSD/Bitcoin`.

Initially I naively moved `/Volumes/SSD/Bitcoin/blocks` to `/Users/bitcoin/blocks` and added a line to `/Volumes/SSD/Bitcoin/bitcoin.conf`: `blocksdir=/Users/bitcoin/blocks`.

I then realized that I shouldn't have moved the `index` subdirectory, so I moved that back to `/Volumes/SSD/Bitcoin/blocks/index`. Having an empty `blocks` dir with just and `index` subdir is a bit weird. Perhaps after @jimpo's #11857 is merged, we could just move / assume all indexes to be there (if `-blocksdir` is set)?

Worse, it created a directory `/Users/bitcoin/blocks/blocks`, which seems like a bug. I had to set `blocksdir=/Users/bitcoin` instead.
fReindex = gArgs.GetBoolArg("-reindex", false);
    bool fReindexChainState = gArgs.GetBoolArg("-reindex-chainstate", false);
    
    int64_t nTotalCache = (gArgs.GetArg("-dbcache", nDefaultDbCache) << 20);
    nTotalCache = std::max(nTotalCache, nMinDbCache << 20); 
    nTotalCache = std::min(nTotalCache, nMaxDbCache << 20); 
    int64_t nBlockTreeDBCache = std::min(nTotalCache / 8, nMaxBlockDBCache << 20);
    nTotalCache -= nBlockTreeDBCache;
    int64_t nTxIndexCache = std::min(nTotalCache / 8, gArgs.GetBoolArg("-txindex", DEFAULT_TXINDEX) ? nMaxTxIndexCache << 20 : 0);
    nTotalCache -= nTxIndexCache;
    int64_t nCoinDBCache = std::min(nTotalCache / 2, (nTotalCache / 4) + (1 << 23)); 
    nCoinDBCache = std::min(nCoinDBCache, nMaxCoinsDBCache << 20); 
    nTotalCache -= nCoinDBCache;
    nCoinCacheUsage = nTotalCache; 
    int64_t nMempoolSizeMax = gArgs.GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000;
    LogPrintf("Cache configuration:\n");
    LogPrintf("* Using %.1fMiB for block index database\n", nBlockTreeDBCache * (1.0 / 1024 / 1024));
    if (gArgs.GetBoolArg("-txindex", DEFAULT_TXINDEX)) {
        LogPrintf("* Using %.1fMiB for transaction index database\n", nTxIndexCache * (1.0 / 1024 / 1024));
    }
    LogPrintf("* Using %.1fMiB for chain state database\n", nCoinDBCache * (1.0 / 1024 / 1024));
    LogPrintf("* Using %.1fMiB for in-memory UTXO set (plus up to %.1fMiB of unused mempool space)\n", nCoinCacheUsage * (1.0 / 1024 / 1024), nMempoolSizeMax * (1.0 / 1024 / 1024));
    bool fLoaded = false;
    while (!fLoaded && !ShutdownRequested()) {
        bool fReset = fReindex;
        std::string strLoadError;
        uiInterface.InitMessage(_("Loading block index..."));
        LOCK(cs_main);
        do {