Synology FlashStation FS6400 review

The Synology FlashStation FS6400 is an older-generation 2U rackmount NAS whose distinction is 24 positions for 2.5-inch SAS hard drives/SSDs and SATA SSDs, plus separate Synology expansion shelves. Synology still lists it as generally available with full DSM updates and technical support. For a team already designed around those media and expansion options, that may matter more than a newer processor generation.

Built for shared file and block storage

The FS6400's 24 hot-swappable 2.5-inch bays support SAS HDDs, SAS SSDs and SATA SSDs, unlike the SATA-SSD-only FS6420. Still check each disk against the current FS6400 compatibility list. SMB/NFS file services, iSCSI/Fibre Channel, Btrfs/ext4 and Virtual Machine Manager point to a business role such as shared project data, virtualization or databases, not a casual desktop appliance.

Synology lists two Intel Xeon Silver 4110 processors, eight cores each, and 32 GB of DDR4 ECC RDIMM. Its page advertises more than 240,000 iSCSI 4K random-write IOPS, an internal manufacturer benchmark rather than an expected file-transfer speed. The actual path and workload can change performance.

Networking and expansion are not the same thing

Built-in networking is two 10GbE RJ-45 plus two 1GbE ports. Its maximum single built-in link is 10GbE; the two faster ports do not automatically make one 20Gb/s client connection. Optional PCIe 3.0 x8 cards add network options. A separate Mini-SAS HD port connects compatible shelves: Synology lists two RX1222sas units for up to 48 total drives or two FX2421 for up to 72. Those separately purchased shelves are not native bays or included capacity.

This shelf path distinguishes FS6400 from FS6420, but requires checking the exact shelf, cables, drives and current topology. Budget rack space, power and cooling for added equipment too.

Redundancy is useful, not a backup

The FS6400 lists RAID 1, RAID 5, RAID 6, RAID 10 and Synology RAID F1, among other options. A two-drive RAID 1 set is therefore a documented way to keep a mirror against one drive failure. It does not protect against accidental deletion, malware, a failed enclosure, stolen equipment or damage to the site. Synology also lists Btrfs snapshots, Snapshot Replication and High Availability; those features can improve recovery and service continuity when they are configured and operated correctly. Pair them with an independent backup destination and a restore test rather than using RAID as a substitute for a second copy.

The 1 PB single-volume option is listed only for RAID 6 with 64 GB memory; the installed 32 GB is below that condition. A buyer relying on the headline needs a supported upgrade and DSM prerequisite check. Drive count, RAID and filesystem overhead mean raw and usable capacity differ.

Installation and package checks

The chassis is 2U, 88 × 482 × 724 mm and 17.26 kg before disks. It requires a four-post 19-inch rack; RKS-02 rails are sold separately. Synology lists 100–240 V AC and redundant power supplies. Its 53.7 dB(A) figure comes from the stated manufacturer test setup, not our measurement; plan a suitable server location rather than assuming home-office quiet.

The hardware guide lists two cords and 100 drive screws but no disks; Synology says drives are sold separately. Budget compatible disks, rails, cards, shelves, memory and installation separately, and verify the seller package rather than relying on a generic “24-bay NAS” listing.

How it compares with the newer FS6420

The FS6420 moves to one 16-core AMD EPYC 7303, 64 GB installed memory, 1 TB maximum memory and two PCIe 4.0 x16 slots, but its listed internal drive media are 2.5-inch SATA SSDs and no matching external shelf expansion is documented. The FS6400 instead has two older 8-core Xeon processors, 32 GB installed memory, 512 GB maximum memory, PCIe 3.0 x8 slots and the SAS/SATA plus shelf options described above. Both are 24-bay, 2U systems with 10GbE, Btrfs, RAID 1 and a five-year warranty listing. These differences matter more than the shared family name: choose around the drive inventory, interface, expansion design and support window, not a processor slogan alone.

Who should evaluate it?

Consider it for a rack-based business NAS needing 24 positions, SAS support, redundant networking or shelf expansion—with staff for DSM and storage administration. Synology lists current availability and full support, but confirm regional terms. A household or small office needing only shared folders will likely gain little from the chassis and deployment work. For a SATA-SSD-only system with no shelf plan, compare the newer FS6420.

Checks before ordering

Identify each proposed disk by its full model number and verify it in the FS6400 compatibility list. Check whether the plan depends on SAS, SATA, a 1 PB volume, 2U rack rails or a specific expansion shelf, then confirm those conditions against Synology's current documents. Verify switch and client support for the intended 10GbE path, rack depth and cooling, the loaded chassis weight and the seller's exact diskless or bundle contents. Prices, stock and a particular system's performance are not established by this documentation-based review.