You can either do LVM snapshots (with journaling) on the ephemeral drives, or use mongodump with the oplog option to get consistant "hot" backups. The downside

You can either do LVM snapshots (with journaling) on the ephemeral drives, or use mongodump with the oplog option to get consistant "hot" backups. The downside of mongodump is it churns your working set.