Synology DS220+ Specs and Usable Capacity

Published specifications for the Synology DS220+, a 2020 model, together with the number the manufacturer page will not give you: how much usable capacity you actually get from 2 bays under each storage layout, at every drive size worth buying. The two-bay that put Quick Sync transcoding in reach of first-time buyers. Discontinued, but common and cheap secondhand.

Synology DS220+ specifications

Specifications come from manufacturer published data. Vendors sometimes ship the same model number with different memory fitted or drives included, so confirm against the listing for the exact unit you are buying. Where a figure is not published, the row is omitted rather than estimated.

How much usable storage does the DS220+ actually give you?

Raw capacity is bays multiplied by drive size. Usable capacity is what survives after parity, and it is always a whole number of drives smaller. Every figure below is computed from 2 identical drives using the same engine behind our cross-vendor RAID capacity calculator, so you can check it.

All capacities are decimal terabytes, the unit printed on the drive label. Your NAS interface will report binary tebibytes and label them "TB", which makes every figure look about 9.05 percent smaller. Nothing is missing: one TB is 1,000,000,000,000 bytes and one TiB is 1,099,511,627,776, so the ratio is a fixed 0.909495. See the TB against TiB chart for the conversion at every capacity, and expect the filesystem to take a further one to two percent on top.

What drives should you put in a DS220+?

The enclosure is the cheap part. 2 drives at $200 to $500 each is a $400 to $1,000 purchase, and unlike the box the drives get replaced and upgraded over the life of the array. Two requirements are not negotiable: CMR recording, never SMR, because shingled drives can take days to rebuild an array, and a published NAS workload rating of at least 180 TB per year.

Buy all the drives at once if you can afford it, so they age together and you know their history. Buying them from different retailers or in two batches is a cheap way to avoid every drive coming from the same manufacturing run. See our NAS drive comparison for the full field and how to choose NAS drives for the reasoning.

What are the real limits of a 2 bay NAS?

A two-bay mirror gives you one drive of usable capacity and survives one failure. That is genuine protection and it is the correct trade at this size, because the alternative at two bays is no protection at all. What it costs you is half of everything you buy: two 16 TB drives yield 16 TB usable, not 32.

The limit that catches people is not capacity, it is the upgrade path. A mirror does not grow. There is no bay to add a drive to, so the only way to increase capacity is to replace both drives with larger ones, rebuild, and find somewhere to put the old pair. Compare that with four bays, where the same money spread across more drives gives three drives of usable capacity instead of one and leaves room to expand. Per terabyte of protected storage, four bays is markedly cheaper, which is worked through on best 4-bay NAS.

SHR changes the upgrade story slightly on this unit but not as much as people expect. With two identical drives SHR produces exactly the same mirror as RAID 1, so it gains you nothing on day one. Its value appears when you replace one drive with a larger one: the extra space becomes usable once the second drive matches it, whereas standard RAID truncates both members to the smaller drive and strands the difference permanently.

Two bays is the right choice when the data set is stable, the budget is genuinely constrained, or the unit is a backup target for a larger array elsewhere. It is the wrong choice when the library is growing, because the upgrade you will want in two years is a bigger box rather than bigger drives. Check your own drive set against every layout on the RAID capacity calculator before committing.

How long does a rebuild take on a 2 bay NAS?

Rebuild duration is set by drive size, not by array size. Reconstructing a failed member means writing a whole replacement drive while reading the survivors, so a larger drive means a longer window during which the array is exposed. At a realistic 80 to 150 MB per second, which is what a mechanical array manages while still serving files, a 16 TB member takes 29.6 to 55.6 hours.

Read volume is computed for 2 identical drives under SHR. ZFS resilvers only the blocks actually in use, so a half-full pool finishes considerably sooner, while a traditional mdadm rebuild reads every sector regardless. Full detail on the rebuild time chart.

Who the Synology DS220+ suits

• Used buyers who want DSM and hardware transcoding for very little money.
• A backup target for a larger NAS.
• Not a long-term purchase. Check how many DSM versions it will still receive before buying.

What it costs to leave running

A NAS runs continuously, so the electricity bill is a real part of ownership. Budget roughly 6 to 9 watts per spinning drive at idle plus the enclosure itself, which for 2 drives lands somewhere around 33 watts idle once the drives have spun up. Put your own tariff and drive count into the power draw calculator for an annual cost, and size a battery with the UPS runtime calculator, because an unclean shutdown during a write is one of the few things that can damage an array rather than a drive.

What else does a DS220+ build need?

Three things get left out of almost every plan at this stage, and all three are cheap relative to what has already been committed to drives.

• A UPS with a data cable. A NAS losing power part way through a write is one of the few events that damages an array rather than a drive, and a unit with no USB connection to the server is a delay rather than a protection: it holds the array up and then drops it mid-write anyway. Size the battery on the UPS runtime calculator, and note that you are sizing for a clean shutdown of 10 to 15 minutes rather than for riding out the outage. Picks are on best UPS for a NAS.
• A backup target. Whatever parity this box runs, the array is copy one. Parity rebuilds a failed drive and does nothing about accidental deletion, ransomware, a controller writing corruption to every member, theft or fire, because each of those reaches every drive in the chassis at once. Work out the capacity on the 3-2-1 backup sizing calculator.
• A network that can keep up. 2 x 1GbE is the published port configuration here. A gigabit link carries about 113 MB per second and a single 7200 rpm drive already sustains 150 to 260, so if this unit has multi-gigabit ports and your switch does not, you have paid for a link speed you never see. Check the arithmetic on the network throughput calculator.

Related reading

• Cross-vendor RAID capacity calculator, SHR against ZFS against standard RAID for the same drives
• Best NAS hard drives, where the money in this build actually goes
• RAID levels explained in plain language
• NAS against a DIY server, if you are still deciding
• All NAS model specifications