We limit the number of concurrent blob downloads per node and per recovery, but if a user configures too many concurrent recoveries they may run out of per-node download permits which yields the following message:

> Unable to acquire permit to use snapshot files during recovery, this recovery will recover index files from the source node. Ensure snapshot files can be used during recovery by setting [indices.recovery.max_concurrent_snapshot_file_downloads] to be no greater than [25]

The `[25]` here is wrong, the 25 permits are shared across all inbound recoveries so we should divide the suggested value by `cluster.routing.allocation.node_concurrent_incoming_recoveries`. In extreme cases the user may have configured so many concurrent recoveries that the only reasonable limit is zero, and in that case we should suggest setting `indices.recovery.use_snapshots: false` instead.


server/src/main/java/org/elasticsearch/indices/recovery/RecoverySettings.java
clusterSettings.addSettingsUpdateConsumer(
            INDICES_RECOVERY_MAX_CONCURRENT_SNAPSHOT_FILE_DOWNLOADS,
            this::setMaxConcurrentSnapshotFileDownloads
        );
        clusterSettings.addSettingsUpdateConsumer(
            INDICES_RECOVERY_MAX_CONCURRENT_SNAPSHOT_FILE_DOWNLOADS_PER_NODE,
            this::setMaxConcurrentSnapshotFileDownloadsPerNode
        );
    }
    private void computeMaxBytesPerSec(Settings settings) {
        
        final long defaultBytesPerSec = Math.max(INDICES_RECOVERY_MAX_BYTES_PER_SEC_SETTING.get(settings).getBytes(), 0L);
        
        final long networkBandwidthBytesPerSec = Math.max(availableNetworkBandwidth.getBytes(), 0L);
        
        final long readBytesPerSec;
        if (availableDiskReadBandwidth.getBytes() > 0L && networkBandwidthBytesPerSec > 0L) {
            double readFactor = NODE_BANDWIDTH_RECOVERY_FACTOR_READ_SETTING.get(settings);
            readBytesPerSec = Math.round(Math.min(availableDiskReadBandwidth.getBytes(), networkBandwidthBytesPerSec) * readFactor);
        } else {
            readBytesPerSec = 0L;
        }
        
        final long writeBytesPerSec;
        if (availableDiskWriteBandwidth.getBytes() > 0L && networkBandwidthBytesPerSec > 0L) {
            double writeFactor = NODE_BANDWIDTH_RECOVERY_FACTOR_WRITE_SETTING.get(settings);
            writeBytesPerSec = Math.round(Math.min(availableDiskWriteBandwidth.getBytes(), networkBandwidthBytesPerSec) * writeFactor);
        } else {
            writeBytesPerSec = 0L;
        }