Synology RackStation RS4021xs+ review: A familiar face

Synology’s speedy high-capacity rack NAS gets a new name and more memory - but no upgrades

Synology RackStation RS4021xs+ front and rear

IT Pro Verdict


  • +

    High storage capacity

  • +

    Excellent 10GbE performance

  • +

    Good expansion potential

  • +

    Great range of data protection apps


  • -

    Virtually identical to the older RS4017xs+

With support for up to 16 LFF drives, the RackStation RS4021xs+ is one of Synology's largest NAS appliances. This 3U rack mounter is aimed at SMBs and enterprises that want a high capacity, general purpose network storage repository and the '+' in its model name indicates that performance is a priority.

The RS4021xs+ is indeed one of Synology's more powerful models, sporting a meaty eight-core 2.1GHz D-1541 Xeon CPU. This is partnered by 16GB of DDR4 ECC UDIMM memory which can be increased to a very usable 64GB.

Synology doesn't explicitly state that it's a replacement for the three-year old RS4017xs+ - and for good reason, as they're virtually identical. Apart from the new name badge, the only physical difference is the RS4021xs+ comes with twice as much base memory as the RS4017xs+.

Synology RackStation RS4021xs+ review: Storage

When populating the appliance with storage devices, pay very close attention to its device compatibility list. As one of Synology's enterprise models, the RS4021xs+ comes under its new verified drive restrictions and the list, unsurprisingly, consists mainly of its recently announced HAT5300 HDDs and SAT5200 SSDs.

The HAT5300 drives are available in 8TB, 12TB and 16TB capacities, are manufactured by Toshiba and come preloaded with Synology's firmware. The list includes a few low-capacity 4TB HDD models from Seagate, Toshiba and Western Digital while SATA SSDs are restricted to four SAT5200 models up to 3.84TB.

The cynical may say Synology is attempting to generate more revenue but from a support perspective, it makes a lot more sense. Using tested and verified drives makes it considerably easier for Synology to provide enterprise customers with a full support service, it avoids the blame game when storage devices go bad and the DSM Storage Manager app provides a handy automatic drive firmware update service.

Synology RackStation RS4021xs+ storage management

The new restrictions don't stop unverified drives being used, but you'll probably wish you hadn't. DSM will allow storage pools and volumes to be created but the Storage Manager app will issue multiple warnings that the appliance is in danger, and its SMART testing and reporting service will be disabled for unverified drives.

Synology RackStation RS4021xs+ review: Hardware features

The appliance is very well built with three 8cm diameter hot-plug cooling fans under a separate, removable cover in the lid. Power redundancy is covered nicely as the price includes two hefty 800W hot-plug PSUs.

Port permutations are the same as the RS4017xs+, with the rear panel presenting quad Gigabit, dual 10GBase-T and a pair of USB 3 ports. Expansion potential looks good, too; the two high-performance Infiniband ports can each be used to attach a 12-bay RX1217/RP disk shelf, pushing the drive count to 40.

Two spare PCI-Express slots are up for grabs and support Synology's own 10GbE and 25GbE adapters, along with a few industry standard Intel and Emulex models. The appliance doesn't have any embedded M.2 slots on its motherboard but you can add Synology's E10M20-T1 10GbE/dual NVMe SSD combo card or the E2D20 dual NVMe SSD card.

Synology RackStation RS4021xs+ review: Software features

With Synology's feature-rich DSM software at the helm, the RS4021xs+ can tackle just about any network storage related task you set it. The Virtual Machine Manager app can run any OS you care to name, including virtualized versions of DSM with the price including one extra license for the latter.

Synology RackStation RS4021xs+ backup manager

Security monitoring is on its agenda as well, and the slick Surveillance Station app transforms the appliance into a sophisticated recording vault, with support for over 7,600 IP camera models and a huge range of video management services. Collaboration between remote workers is also facilitated by the Office app suite, which is offered as a free alternative to Microsoft 365 and provides a central location for users to create and securely share documents.

Unsurprisingly, you're spoilt for choice with backup services. DSM protects on-appliance data with scheduled snapshots and remote replication while the Hyper Backup app manages all local, remote, Rsync, cloud and iSCSI LUN backups from a single console. Servers and workstations (along with VMware and Hyper-V virtualization hosts) are looked after by the free Active Backup for Business (ABB) app which applies deduplication to its backup vault for big storage savings. ABB is part of the Active Backup Suite, which also provides separate apps for securing Microsoft 365 and Google Workspace.

Synology RackStation RS4021xs+ review: 10GbE performance

For testing, we configured four 8TB HAT5300 disks in a RAID5 array and called up the lab's Dell EMC PowerEdge T640 Xeon Scalable tower running Windows Server 2019 for host duties. NAS performance is impressive, with a share mapped to the server over 10GbE delivering Iometer raw sequential read and write speeds both of 9.3Gbits/sec.


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Real world performance was equally good; copies of a 25GB test file delivered average read and write rates of 5.3Gbits/sec and 5.1Gbits/sec, while our backup test secured a 22.4GB test folder and 10,500 small files at 2.6Gbits/sec. The Xeon's encryption engine is very efficient too, and copying the 25GB test file to an encrypted folder averaged a speedy 2.7Gbits/sec.

IP SAN performance won't disappoint either, with one 10GbE link to a 1TB target returning Iometer read and write speeds both of 9.3Gbits/sec. To push the appliance harder, we fitted a Synology dual-port 10GbE adapter in the appliance, configured a quad 10GbE MPIO to the target and saw Iometer report high read and write rates of 36.6Gbits/sec and 17.7Gbits/sec.

Synology RackStation RS4021xs+ review: Verdict

With nothing more than a name change and a doubling in memory, there isn't anything here to tempt existing RS4017xs+ users to upgrade. That said, the RS4021xs+ is a tempting proposition for SMBs and larger businesses that want an affordable high-capacity rack NAS offering top 10GbE performance, a remarkable range of storage apps and a generous 5-year warranty.

Synology RackStation RS4021xs+ specifications

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Chassis3U rack
CPU2.1GHz 8-core Intel Xeon D-1541
Memory16GB DDR4 ECC UDIMM (max 64GB)
Storage16 x SATA LFF/SFF
RAIDRAID0, 1, 10, 5, 6, F1, JBOD
Expansion2 x PCI-E 3
Network4 x Gigabit, 2 x 10GBase-T
Other ports2 x USB 3, 2 x Infiniband
Power2 x 800W hot-plug PSUs
ManagementWeb browser
Warranty5 years limited
Dave Mitchell

Dave is an IT consultant and freelance journalist specialising in hands-on reviews of computer networking products covering all market sectors from small businesses to enterprises. Founder of Binary Testing Ltd – the UK’s premier independent network testing laboratory - Dave has over 45 years of experience in the IT industry.

Dave has produced many thousands of in-depth business networking product reviews from his lab which have been reproduced globally. Writing for ITPro and its sister title, PC Pro, he covers all areas of business IT infrastructure, including servers, storage, network security, data protection, cloud, infrastructure and services.