10 GbE NAS turned out five times slower than expected — two mirrored drives are to blame

Moving to a 10-gigabit home network does not guarantee the expected speed boost: real throughput can end up five times lower than theoretical figures, and the network gear is not the culprit.

A user who decided to upgrade their storage setup installed a managed switch and a Synology DS1621xs+ NAS with a built-in 10 GbE port. Calculations suggested that virtual machines and backups would transfer at 1,250 MB/s, but benchmarks showed just 230 MB/s. The cause turned out to be the disk array configuration, not the network.

The storage unit held two 20 TB drives in SHR, which is effectively RAID 1. That setup provides redundancy, but every write goes to both disks, capping the maximum data transfer rate at roughly 290 MB/s. That is barely more than 20% of what a 10 GbE link can handle.

Read operations were an even bigger letdown. The user assumed two copies of data would let both drives work in parallel, but the Linux kernel underneath Synology has run sequential reads from only one mirror member for two decades. Load balancing only kicks in when several clients pull data simultaneously — a single person copying a large file gets the speed of one drive.

A test using 64 GB of direct I/O over SSH, with no caching or network overhead, showed 259 MB/s sustained writes and 274 MB/s sustained reads. The array was only 6% full at the time, which represents the best performance it will ever see: as the drives fill up, speeds drop due to slower inner tracks.

The SMB protocol can add its own limits. Copying a single file is handled by one TCP stream on one CPU core, making each connection single-threaded. SMB Multichannel could break that constraint, but it only arrived in DSM version 7.2, and this NAS has just one 10 GbE port.