Synology DS920+ Specs and Usable Capacity

Published specifications for the Synology DS920+, a 2020 model, together with the number the manufacturer page will not give you: how much usable capacity you actually get from 4 bays under each storage layout, at every drive size worth buying. For several years the default recommendation for a home NAS, because it combined four bays, Quick Sync transcoding and DSM at a sane price.

Synology DS920+ 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.

On the M.2 slots: NVMe cache.

How much usable storage does the DS920+ 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 4 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.

Which RAID level should you run on a 4 bay NAS?

Because this is a Synology, you get SHR and SHR-2 as well as the standard levels. With identical drives SHR gives exactly the same capacity as RAID 5 and SHR-2 the same as RAID 6, so there is no penalty for choosing them. The difference appears later, when you replace one drive with a larger one: SHR uses the extra capacity, and RAID 5 strands it.

Past 8 to 10 TB per drive, run double parity. With 20 TB drives in this chassis a single-parity rebuild must read roughly 60 TB from the surviving members. At a realistic 80 to 150 MB per second that is well over a day of continuous reading across drives that are the same age and often from the same batch, and it puts about a 38 percent chance of meeting an unrecoverable read error against the rated floor for NAS drives. Treat that as a way to rank layouts, not a forecast: the rating is a warranty floor rather than a measurement, and modern systems survive a single read error rather than aborting.

Run your own drive set through the RAID capacity calculator to see the same comparison with mixed sizes, and the drive failure probability calculator to put annualised failure rates against 4 drives over several years.

What drives should you put in a DS920+?

The enclosure is the cheap part. 4 drives at $200 to $500 each is an $800 to $2,000 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.

How long does a rebuild take on a 4 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 4 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.

The reason that window matters is correlation. During a rebuild every surviving member is read end to end, and those members are the same age, ran the same hours and frequently came from the same manufacturing run, which is the condition under which a second failure is least independent. Split the drive order across two retailers or two batches, and run double parity once the drives pass roughly 8 to 10 TB. Put your own numbers through the drive failure probability calculator.

Who the Synology DS920+ suits

• Used buyers who want a four-bay Plex-capable DSM box.
• Anyone replacing a failing older two-bay without changing ecosystems.
• Not a new purchase. The DS423+ is the current equivalent.

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 4 drives lands somewhere around 48 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 DS920+ 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