Synology RS4021xs+review. An inexpensive NAS that has everything!

In the Synology corporate storage line, the RS40xx series is the main workhorse, on the basis of which you can solve most tasks in small and medium-sized businesses. This model has the most versatile form factor with 16 HDD bays, the ability to install NVMe and SATA SSD caches, two 10-Gigabit network ports and a modern 8-core Intel Xeon D-1541 processor with high energy efficiency.

As a rule, customers choose such devices for systems associated with large amounts of data: for storing archives, video surveillance systems, backup or general file sharing needs. Synology nass perform such tasks independently using the built-in software, so that the customer who purchases Synology RS40xx already understands which services he will consolidate on the NAS using Synology software packages, and which (for example, under Windows) will be able to run in the Synology Virtual Machine Manager virtual machine or in the form of containers in Docker. Perhaps today it is more correct to refer Synology RS devices to software and hardware complexes that perform a wide range of tasks, but it is also fair to call them the old-fashioned NAS, servers or storage. Today we are testing the RS4021XS+ model, a 2021 model with a fault-tolerant power supply.


Since the RS4021XS+ model in question belongs to the main class of devices for business, Synology has not experimented with the design of devices. There are no bold technical solutions, no controversial points, and the whole focus of attention is shifted to clarity and energy efficiency. From the front part, 16 3.5-inch compartments are placed on the 3U case. It supports the installation of hard drives and SSDs with the SATA interface.

If you plan to use the SSD cache in the device, then in order not to occupy the 3.5-inch compartment, you can install the Synology M2D20 card, an adapter for two NVMe SSDs. This solution is good for everyone, except that it does not support hot swapping for M. 2 type drives, so if non-stop business is a priority, it is better to use a SATA SSD.

To cool the components, three 80-mm hot-swappable fans are located in the central part of the case, access to which is possible without disassembling the entire server case. In general, thanks to the use of a SoC processor from the Xeon-D series, Synology RS4021XS+ ' s own power consumption is very low, and the device can be installed in server racks without special requirements for air conditioning or noise insulation.

In the factory configuration, the Synology RS4021XS+ has 1x 16 GB RAM module installed. When using virtualization, you may need an additional amount of memory - in total, the system supports up to 64 GB (4 modules of 16 GB each) of RAM.

According to the connection interfaces, the situation is as follows: RS4021xs+ in the basic configuration has only "copper" ports for a twisted pair: 4 pieces with a speed of 1 Gbit/s and 2 with a speed of 10 Gbit / s. Synology has proprietary expansion boards with various interfaces in its arsenal:

  • 10GBase-T + 2 NVMe M combo board.2 slots
  • Network adapters with SFP + slots with support for 10 and 25 Gbps
  • Multi-gigabit network adapters 2.5/5/10 Gbps, which are usually used in Wi-Fi 6 and IoT deployment networks

To expand the capacity of the device, it is allowed to connect up to two RX1217/RX1217RP disk shelves via the Infiniband interface. Each of the shelves can be connected with a single port, without duplication for bus fault tolerance. In total, up to 40 SATA drives can be connected to one Synology RS4021xs+ head unit, taking into account disk shelves.

Synology HAT5300

The device came to us for testing in a complete set with Synology HAT5300 hard drives. Previously, we have repeatedly stressed that Synology NAS's do not have the so-called Vendor Lock protection for components, and are compatible with a wide range of desktop and server hard drives, but times are changing, and modern HDDs have become much more complex than models of 5 or 10 years ago. No, Synology has not lost the advantages of wide compatibility, but also began to offer hard drives under its own brand. It goes without saying that these HDDs are at the top of the compatibility list, but this is not their only plus.

  • First, Synology uses Enterprise-grade products for delivery under its own name: Toshiba makes disks for the company, which is considered a very reliable supplier.
  • Secondly, the firmware of hard drives is updated along with the DSM, and this point should not be neglected. For example, in branded storage systems for 2.5-inch SAS HDD for a service period of 3-5 years, 3-4 firmware updates could be issued for disks, and with a note for urgent installation. Updating 20-40 disks in one storage without the support of this function from the storage itself is a very time - consuming task, and in this regard, Synology is well done.
  • Thirdly, the company provides a 5-year warranty for its hard drives, and this, it is worth admitting, is the most noticeable of all the advantages.

As for performance, yes, today there are certain technologies to increase speed in multithreaded tasks inherent in storage. This is usually achieved through read caching algorithms, but in some cases, vibration sensors can also be used to correct the positions of the disk heads in RAID arrays. It is not specified what technologies Synology used to achieve advantages over competitors, but the statement that "our disks work 23% faster under the same conditions" is a good argument even if you use an SSD cache.

From the technical side, these are completely ordinary HDD at 7200 RPM with a cache capacity of 256 MB for 8 and 12 Tb models or 512 MB for 16 Tb models. Even the default sector size is 512 bytes.


At the time of preparing the review, only DSM version 6.2 was available for the RS4021xs+, although the 7th version with very significant changes has already been released for many Synology models. In our testlab, we have tested DSM 6.2, as they say, inside and out, and we offer you to get acquainted in detail with the interesting features of Synology for business:

Active backup for Business (see the review) - one of the most useful, in my opinion, features of Synology is the ability to delegate the management of backups to the NAS. The NAS itself configures windows for backup, it deduplicates copies to save space, it backs up virtual machines, working computers and servers based on Linux/Windows, as well as any network folder over the SSH protocol. In case of an accident, physical and virtual machines can be restored both to the original location on the server, and to the storage that is quickly created on the NAS, or even to its own virtualization system on it. Our record for the VM recovery speed = 11 seconds!

As for the built-in virtualization tools, Synology allows you to build fault-tolerant clusters (see the review), in which there is no drop in the speed of the disk system, as in software-defined storage. The whole process is very convenient and intuitive.

But in addition to virtual machines, Synology RS4021xs+ can also back up data stored in Google and Microsoft cloud services using Active Backup for G Suite and Active Backup for Office 365 packages (see the review). A typical problem of users, when something has disappeared from the cloud, or access to the service has disappeared due to government restrictions, is not terrible with these solutions - copies of documents are stored locally.

Unfortunately, the Active Backup series packages do not support saving data to the cloud, so Synology's 1-2-3 backup method is currently available only for local resources using the Hyper Backup utility. Synology is actively promoting its own C2 cloud service, through which it plans to provide advanced features for customers. To date, password storage (C2 Password) and data storage (C2 Storage) have been launched within Synology C2. You can connect several NAS-s to one account for more efficient use of space.

The easiest way to control the protection of working folders is through a system of snapshots, since this is a native function for the BTRFS file system. To save space and increase reliability, snapshots can be replicated to a third-party NAS through the Snapshot Replication application.

At any time, you can estimate the cost of free space for storing snapshots, and if necessary, delete the old versions. And yet, do not forget that snapshots, even if stored on another server, do not replace full-fledged backups.

File protocols

It is assumed that one Synology NAS can cover all the needs for file-sharing protocols: SmB versions 1-3 are supported for creating shared resources on the network, NFS for connecting, for example, VMware or OpenStack storage, AFP for old Apple computers and iSCSI for universal block access.

For Thin volumes, the function of snapshots with replication to a third-party device and a configurable storage policy is supported, where, for example, you can specify the storage of 256 snapshots with cyclic deletion of old ones.

SSD caching

The SSD cache can operate in two modes: read-only and read/write. In the first case, 1 solid-state drive is enough for you, and in the second case, you will need at least a pair to combine into a "mirror". Caching SSDs can also be combined into more complex arrays, including RAID 5, the main thing is that fault tolerance is provided for caching records.

In the current version of DiskStation Manager, the contents of the SSD read cache are not saved after restarting the NAS, that is, after restarting you will have a certain warm-up period, although DSM starts saving data to the SSD literally from the first minutes after startup. As for read/write caching, the behavior here is completely predictable: the contents of the cache are saved even after a reboot.

We will test the speed of filling the SSD cache, for which we will first use 1 SSD in read cache mode, and then 2 SSDs in read/write cache mode, combining them into a mirror RAID 1. We can see that the SSD significantly speeds up the performance of the disk system, and the mirror array additionally increases performance by reading from two drives simultaneously. But in addition to the fact that the SSD cache works faster, it also fills up faster, which is clearly seen in the following logarithmic diagram.

It turns out that SSDs reach the maximum speed after 3-4 minutes, and in addition, the SSD cache releases data more actively and is rebuilt between loads, and to show this, we use patterns with simultaneous reading and writing. We will use 2 sizes of the test area: 16 GB, comparable to the amount of RAM and 96 GB, three times more than there is memory in the NAS.

Where a record is added, you already need to take a more competent approach to choosing the SSDs themselves, given that most likely they will be constantly filled with data, and their speed will differ from the maximum. Let's increase the test area by 6 times:

By the way, Synology DSM constantly monitors the health of SSD-shek and will warn you when it is better to replace the drive. Let's change the pattern to SQL, and look at the behavior of the array.

Interestingly, in the SQL load, when the cache is filled, the amplitude of performance fluctuations decreases. Let's compare the average values in different patterns.

Tests show that SSD caching can significantly improve performance in real workloads.


Synology RS4021xs+ is a NAS in the form in which it should be today or tomorrow. Even in the basic configuration, its power is sufficient to run such ordinary tasks as a backup station or a mail server. If you plan to launch virtual machines or Docker containers, it will not be superfluous to increase the amount of RAM immediately to 64 GB, since the prices for memory are very affordable today. If each 3.5-inch compartment is needed for HDD, you can install an SSD on the expansion card, leaving another slot for high-speed interfaces.

In principle, the RS4021xs+ is a clear and predictable model that you can choose with an eye to the future release of DSM 7, the capabilities of which we discussed earlier.

Michael Degtjarev (aka LIKE OFF)

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