Should NAS buy two or four slots? I've used both machines, so let me calculate this for you. Don't save first and replace it like I did

who can go straight to four trays, it's more cost-effective in one go

on the other hand, if you get involved in any of the lower points, I advise you to go straight to the fourth disc and not follow my old path:

• have a habit of stockpiling movies, especially 4K original discs—each movie costs 60-90GB, and stocking up on ten movies a month is 1T
• two or three phones at home need to automatically back up photos and videos
• and you need to run virtual machines or heavy Docker (virtual drive files have no storage limit)
• want to treat the NAS as a home download device + video library + surveillance storage

core logic boils down to one sentence: demand will grow, but the market will not . Today you think you only keep photos, six months later a friend recommends Jellyfin, and a year later you want to bring in the surveillance cameras from your hometown. These are all real events happening to me. For dual-bay machines, there are only three options: replace the entire unit (losing the most), split RAID 1 into two separate drives to double capacity but no mirroring (playing with heartbeats), or install an external USB hard drive enclosure (ugly and slow). None of these were as good as spending a few hundred yuan more to buy four trays back then.

price difference for you to consider: Synology DS224+ (dual discs) is just over 3,000 yuan, DS424+ (quad discs) is about 4,500, a difference of about 1,300 yuan; The price difference between the domestically produced JiSpace Z2Pro and Z4Pro is also in the thousands. This 1,000-plus yuan is for double the seating and the next three to five years of comfort. Spread by usage years, it's over 200 yuan a year—worth more than any accessories.

The real cost of four drives: electricity, noise, and location

Of course, upgrading four trays isn't for nothing. The cost is clear—don't just listen to good words.

electricity bill is the main part. A mechanical hard drive typically costs 5-8W on standby, and 8-10W during read/write mode. Four drives consume just over 10W more power than two, and if the drive runs continuously throughout the year, it consumes about 88 kWh of electricity. At 60 yuan per kWh, that's over 50 yuan per year. Not much, but if you're on an enterprise drive (like Galaxy Exos, with one unit at 12W or more), then the electricity cost and noise for four yuan should be carefully considered. My current plan is to keep two Coolwolf 8T slots in the commonly used pool, leaving the remaining two slots empty— slots can be left empty for needs. This is the best use for four slots, it's not about maxing them out to be worth it.

noise, watch the market and the machine. NAS drives (Cool Wolf, Western Digital Red) have speed adjustment, so you can barely hear them during idle time; The clicking sound of the corporate development is really unbearable for the bedroom. The case fans are small, and the Synology four-bay fan is basically insensitive at low speed from two meters away at night. It's recommended to place it in a corner of the living room or outside a ventilated area outside the weak current box. Don't stuff it inside a cabinet, as the hard drive's lifespan will be shortened if it stays above 45°C for a long time.

a quick note: Should you add SSD cache

Since we're talking about drive slots,

will also address the most common question beginners often ask: "Should I insert two SSDs for caching?" because four-bay models often have two SSD slots inserted, and I think most of the time it's wasteful.

SSD cache accelerates random small file read/write , corresponding to scenarios such as multiple simultaneous access, virtual machine disks, and databases. The biggest headache for home users is sequentially copying large files (loading movies or backup photos into the NAS) because mechanical drives have already run the gigabit network sequentially; the bottleneck lies in the network port, not the disk. I tested it: the DS220+ added two 500G NVMe chips for read/write cache, and the speed of copying movies was exactly the same as before, both running at around 110MB/s with gigabit speed. Later, I converted those two M.2 slots into separate storage pools for Docker containers and virtual machines, which turned out to be much more practical—the container logs were spent on small files and sent to the SSD, while the mechanical drive focused on storing big data, each doing their own thing.

so my approach is: use the NAS as a virtualization host or if there are spare SSD slots, treat the SSD as a dedicated storage pool, not as a cache; If you're just storing photos or videos, save that money and upgrade your network card or switch to a 2.5G switch, with an immediate improvement in experience—provided your home has broadband and a local network that keeps up. Don't talk about 10 Gbps over gigabit networks—that's another story.

a table to wrap up, and take the right place to sit down