“How much faster will my VM’s disks be if I upgrade my ZFS-based (Z File System) NAS to 10 GbE?” The disks will be faster, in some cases, much faster. Our experience is that sequential read throughput will be 1.4✕ faster, write throughput, 10✕ faster, and IOPS, 1.6✕ faster.

We ran a three-hour benchmark on our NAS server before and after upgrading to 10 GbE. We ran the benchmark again after upgrading. The benchmark looped through write-read-iops tests continuously.

The Sequential Read Results:

We were underwhelmed with the 10 GbE read performance — we felt it should have been better, and surprised that it wasn’t. The 1 GbE results were close to the theoretical network maximum, and we thought the upgrade would have unleashed its full potential. Little did we know that its full potential was barely above 1 GbE.

The lackluster performance wasn’t due to RAM pressure: there was enough RAM on the FreeNAS server (128 GiB) to cache the contents of the sequential read test (16 GiB) several times over, and this was borne out by running zpool iostat 5, which showed the disks were barely touched during the read test.

We also noticed dips in 10 GbE performance at regular intervals (where the read throughput dropped to less than 120 MB/sec); we are not sure what caused these dips, but they seemed to occur almost exactly 35 minutes apart (although timestamps aren’t included in the graph above, they are included in the raw benchmark results, which can be viewed in the References section below).

As an aside, there are fewer 1 GbE tests than 10 GbE tests in the above chart. That is, there are fewer blue dots than red dots, for the 1 GbE tests took longer to run, hence there were fewer results reported within the three-hour test window.

The Sequential Write Results:

The sequential write performance, shown in the above graph, was the star of the show: a 10✕ increase, linear with the network bandwidth increase.

There are some caveats. Most importantly, we sacrificed safety for speed. Specifically, we did not override the default ZFS setting, sync=standard. As pointed out in the article, “Sync writes, or: Why is my ESXi NFS so slow, and why is iSCSI faster?”:

iSCSI by default does not implement sync writes. As such, it often appears to users to be much faster…. However, your VM data is being written async, which is hazardous to your VM

You may ask, “What are these sync writes to which you refer?” Robert Milkowski, the author of the ZFS sync feature, describes it succinctly in his blog post:

Synchronous file system transactions (fsync, O_DSYNC, O_SYNC, etc) are written out [to disk before returning from the system call]

In other words, these are system calls (fsync(2)) or flags to system calls ( open(2)’s ‘O_DSYNC and O_SYNC) to make sure that the data has really, truly been written out to disk before returning from a write(2). When the system call returns, you know that the data’s on the disk, not in some buffer somewhere.

[Editor’s note: this is not always true. Linux usually writes the data to disk, but sometimes it doesn’t. For the dirty details, see this post.]

sync is a great feature when ZFS is used as a fileserver (technically a distributed file system), via protocols such as NFS (Network File System), SMB (Server Message Block), or even the deprecated AFP (Apple Filing Protocol, formerly AppleTalk Filing Protocol):

  • The applications that depend on synchronous behavior (that have opted-in via system call or flag) are guaranteed that their data has been written to disk
  • The majority of applications who don’t need synchronous behavior can take advantage of the high write speeds (often ten times as fast)

But here’s the rub: iSCSI is not a distributed file system. It’s a distributed block device. Which means it’s at a lower layer than NFS, SMB, and AFP. There are no system calls, no directories, no file permissions, no files; there are only reads and writes.

The upshot is that if power is cut to the NAS server while the VM is active, the and the NAS hasn’t yet flushed the outstanding writes to disk, then the VM’s file system may be corrupted beyond the ability of file system repair tools to fix. We witnessed this problem firsthand by cutting the power to the NAS server while actively exercising a Linux VM’s iSCSI disk by running the gobonniego file system benchmark. Our Linux system’s btrfs filesystem was corrupted beyond the ability of btrfsck to fix, emitting cryptic errors such as “child eb corrupted” and “parent transid verify failed” before finally giving up. We lost our VM, and had to reinstall Linux from scratch.

The IOPS Results:

The IOPS results were good, averaging 21k. As a comparison, they were much better than a mechanical hard drive’s (44 -203), and better than even SATA 3 Gbit/s’s 5k-20k, but nowhere near as performant as the higher end NVMe drives (e.g. Samsung SSD 960 PRO’s 360k).



Component10 GbE
(new, 2019)
1 GbE
(old, 2104)
Motherboard$820 1 × Supermicro X10SDV-8C-TLN4F+ Mini-ITX 1.7GHz 35W 8-Core Intel Xeon D-1537$375 1 × Supermicro A1SAi-2750F Mini-ITX 2.4GHz 20W 8-Core Intel C2750
Ethernet controller(built-in)(built-in)
RAM$1,336 4 × D760R Samsung DDR4-2666 32GB ECC Registered$372 4 × Kingston KVR13LSE9/8 8GB ECC SODIMM
HBA(same HBA)$238 1 × LSI SAS 9211-8i 6Gb/s SAS Host Bus Adapter
Disk(same Disk)$1,190 7 × Seagate 4TB NAS HDD ST4000VN000
Power Supply(same Power Supply)$110 1 × Corsair HX650 650 watt power supply
Ethernet Switch$589 1 × QNAP QSW-1208-8C 12-port 10GbE unmanaged switch$18 1 × TP-Link 8-Port Gigabit Desktop Switch TLSG1008D
SFP+ Modules$79 2 × Ubiquiti UF-MM-10G U Fiber SFP+ Module 2-packN/A
Software(same Software)FreeNAS-11.2-U4.1

Virtual Machine:


We didn’t control solely for the 1 GbE → 10 GbE differences: the ethernet controllers were built into the motherboards, hence the ethernet controller upgrade entailed a motherboard upgrade in turn entailing CPU & RAM upgrades.

In other words, the performance increases can’t be attributed solely to the change in network controllers; faster CPUs and larger (32 GiB → 128 GiB) and faster (DDR3 → DDR4) RAM may also have been a factor.

We have seen at least one dramatic speed-up that we could not attribute to the ethernet upgrade: when browsing an AFP (Apple Filing Protocol) share from our WiFi-based laptop, we noticed that one large directory (4.5k entries), which previously took ~30 seconds to display in Apple’s Finder, now displayed in sub-second time. We know the WiFi’s throughput (200 Mb/s) was minuscule compared to the file server’s (1 Gb/s, 10 Gb/s), so the reason for the speed-up lay elsewhere.

Other VMs had disks that were placed on the iSCSI datastore; they may have contended with benchmark VM for disk access. We feel the contention was minor at most.