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OpenZFS is a file system with volume management capabilities designed specifically for storage servers.

Some neat features of ZFS include:

  • Aggregating multiple physical disks into a single filesystem.
  • Automatically repairing data corruption.
  • Creating point-in-time snapshots of data on disk.
  • Optionally encrypting or compressing data on disk.


I've found that learning about ZFS was an interesting, intense and time consuming task. If you want a quick overview check this video. If you prefer to read, head to the awesome Aaron Toponce articles and read all of them sequentially, each is a jewel. The docs on the other hand are not that pleasant to read. For further information check JRS articles.

Storage planning

There are many variables that affect the number and type of disks, you first need to have an idea of what kind of data you want to store and what use are you going to give to that data.


ZFS is designed to survive disk failures, so it stores each block of data redundantly. This feature complicates capacity planning because your total usable storage is not just the sum of each disk’s capacity.

ZFS creates filesystems out of “pools” of disks. The more disks in the pool, the more efficiently ZFS can use their storage capacity. For example, if you give ZFS two 10 TB drives, you can only use half of your total disk capacity. If you instead use five 4 TB drives, ZFS gives you 14 TB of usable storage. Even though your total disk space is the same in either scenario, the five smaller drives give you 40% more usable space.

When you’re building a NAS server, you need to decide whether to use a smaller quantity of large disks or a larger quantity of small disks. Smaller drives are usually cheaper in terms of $/TB, but they’re more expensive to operate. Two 4 TB drives require twice the electricity of a single 8 TB drive.

Also keep in mind that so far ZFS doesn't let you add a new drive to an existing vdev, but that feature is under active development. If you want to be safe, plan your vdev definition so that they don't need to change the disk numbers.

Preventing concurrent disk failures

Naively, the probability of two disks failing at once seems vanishingly small. Based on Backblaze’s stats, high-quality disk drives fail at 0.5-4% per year. A 4% risk per year is a 0.08% chance in any given week. Two simultaneous failures would happen once every 30,000 years, so you should be fine, right?

The problem is that disks aren’t statistically independent. If one disk fails, its neighbor has a substantially higher risk of dying. This is especially true if the disks are the same model, from the same manufacturing batch, and processed the same workloads.

Further, rebuilding a ZFS pool puts an unusual amount of strain on all of the surviving disks. A disk that would have lasted a few more months under normal usage might die under the additional load of a pool rebuild.

Given these risks, you can reduce the risk of concurrent disk failures by choosing two different models of disk from two different manufacturers. To reduce the chances of getting disks from the same manufacturing batch, you can buy them from different vendors.

Choosing the disks to hold data

Check to get an idea of the cost of disks in the market. If you can, try to avoid buying to Amazon as it's the devil. Try to buy them from a local store instead, that way you interact with a human and promote a local commerce.


"If you want a TL;DR you can jump to the conclusion."

To choose your disks take into account:

Data disk speed

When comes to disk speed there are three kinds, the slow (5400 RPM), normal (7200 RPM) and fast (10k RPM).

The higher the RPM, the louder the disk is, the more heat it creates and the more power it will consume. In exchange they will have higher writing and reading speeds. Slower disks expand the lifecycle of the device, but in the case of a failed disk in a RAID scenario, the rebuild time will be higher than on faster ones therefore increasing the risk on concurrent failing disks.

Before choosing a high number of RPM make sure that it's your bottleneck, which usually is the network if you're using a 1Gbps network. In this case a 10k RPM disk won't offer better performance than a 7200 RPM, even a 7200 one won't be better than a 5400.

The need of higher speeds can be fixed by using an SSD as a cache for reading and writing.

Data disk load

Disk specifications tell you the amount of TB/year they support, it gives you an idea of the fault tolerance. Some examples

Disk Fault tolerance (TB/year)
WD RED 8TB 180
Data disk type

It’s easy to think that all hard drives are equal, save for the form factor and connection type. However, there’s a difference between the work your hard drive does in your computer versus the workload of a NAS hard drive. A drive in your computer may only read and write data for a couple hours at a time, while a NAS drive may read and write data for weeks on end, or even longer.

The environment inside of a NAS box is much different than a typical desktop or laptop computer. When you pack in a handful of hard drives close together, several things happen: there’s more vibration, more heat, and a lot more action going on in general.

To cope with this, NAS hard drives usually have better vibration tolerance and produce less heat than regular hard drives, thanks to slightly-slower spindle speeds and reduced seek noise.

Most popular brands are Western Digital Red and Seagate IronWolf which use 5400 RPM, if you want to go on the 7200 RPM speeds you can buy the Pro version of each. I initially tried checking Backblaze’s hard drive stats to avoid failure-prone disks, but they use drives on the pricier side.

The last pitfall to avoid is shingled magnetic recording (SMR) technology. ZFS performs poorly on SMR drives, so if you’re building a NAS, avoid known SMR drives. If the drive is labeled as CMR, that’s conventional magnetic recording, which is fine for ZFS.

SMR is well suited for high-capacity, low-cost use where writes are few and reads are many. It has worse sustained write performance than CMR, which can cause severe issues during resilvering or other write-intensive operations.

There are three types of SMR:

  • Drive Managed, DM-SMR: It's opaque to the OS. This means ZFS cannot "target" writes, and is the worst type for ZFS use. As a rule of thumb, avoid DM-SMR drives, unless you have a specific use case where the increased resilver time (a week or longer) is acceptable, and you know the drive will function for ZFS during resilver.
  • Host Aware, HA-SMR: It's designed to give ZFS insight into the SMR process. Note that ZFS code to use HA-SMR does not appear to exist. Without that code, a HA-SMR drive behaves like a DM-SMR drive where ZFS is concerned.
  • Host Managed, HM-SMR: It's not backwards compatible and requires ZFS to manage the SMR process.
Data disk homogeneity

It's recommended that all the disks in your pool (or is it by vdev?) have the same RPM and size.

Data disk warranty

Disks are going to fail, so it's good to have a good warranty to return them.

Data disk brands
Western Digital

The Western Digital Red series of NAS drives are very similar to Seagate’s offering and you should consider these if you can find them at more affordable prices. WD splits its NAS drives into three sub-categories, normal, Plus, and Pro.

Specs WD Red WD Red Plus WD Red Pro
Technology SMR CMR CMR
Bays 1-8 1-8 1-24
Capacity 2-6TB 1-14TB 2-18TB
Speed 5,400 RPM 5,400 RPM (1-4TB) 7200 RPM
Speed 5,400 RPM 5,640 RPM (6-8TB) 7200 RPM
Speed 5,400 RPM 7,200 RPM (8-14TB) 7200 RPM
Speed ? 210MB/s 235MB/s
Cache 256MB 16MB (1TB)
Cache 256MB 64MB (1TB) 64MB (2TB)
Cache 256MB 128MB (2-8TB) 256MB (4-12TB)
Cache 256MB 256MB (8-12TB) 512MB (14-18TB)
Cache 256MB 512MB (14TB)
Workload 180TB/yr 180TB/yr 300TB/yr
MTBF 1 million 1 million 1 million
Warranty 3 years 3 years 5 years
Price From $50 From $45 From $78

Seagate's "cheap" NAS disks are the IronWolf gama, there are two variations IronWolf and IronWolf Pro. Seagate Exos is a premium series of drives from the company. They’re even more advanced than IronWolf Pro and are best suited for server environments. They sport incredible levels of performance and reliability, including a workload rate of 550TB per year.

Specs IronWolf IronWolf Pro Exos 7E8 8TB
Technology CMR CMR CMR
Bays 1-8 1-24 ?
Capacity 1-12TB 2-20TB 8TB
RPM 5,400 RPM (3-6TB) 7200 RPM 7200 RPM
RPM 5,900 RPM (1-3TB) 7200 RPM 7200 RPM
RPM 7,200 RPM (8-12TB) 7200 RPM 7200 RPM
Speed 180MB/s (1-12TB) 214-260MB/s (4-18TB) 249 MB/s
Cache 64MB (1-4TB) 256 MB 256 MB
Cache 256MB (3-12TB) 256 MB 256 MB
Power Consumption (8TB) 10.1 W 10.1 W 12.81 W
Power Consumption Rest (8TB) 7.8 W 7.8 W 7.64 W
Workload 180TB/yr 300TB/yr 550TB/yr
MTBF 1 million 1 million 2 millions
Warranty 3 years 5 years 5 years
Price From $60 From $83 249$

Where MTBF stands for Medium Time Between Failures in hours

Data disk conclusion

I'm more interested on the 5400 RPM drives, but of all the NAS disks available to purchase only the WD RED of 8TB use it, and they use the SMR technology, so they aren't a choice.

The disk prices offered by my cheapest provider are:

Disk Size Price
Seagate IronWolf 8TB 225$
Seagate IronWolf Pro 8TB 254$
WD Red Plus 8TB 265$
Seagate Exos 7E8 8TB 277$
WD Red Pro 8TB 278$

WD Red Plus has 5,640 RPM which is different than the rest, so it's ruled out. Between the IronWolf and IronWolf Pro, they offer 180MB/s and 214MB/s respectively. The Seagate Exos 7E8 provides much better performance than the WD Red Pro so I'm afraid that WD is out of the question.

There are three possibilities in order to have two different brands. Imagining we want 4 disks:

Combination Total Price
IronWolf + IronWolf Pro 958$
IronWolf + Exos 7E8 1004$ (+46$ +4.5%)
IronWolf Pro + Exos 7E8 1062$ (+54$ +5.4%)

In terms of:

  • Consumption: both IronWolfs are equal, the Exos uses 2.7W more on normal use and uses 0.2W less on rest.
  • Warranty: IronWolf has only 3 years, the others 5.
  • Speed: Ironwolf has 210MB/s, much less than the Pro (255MB/s) and Exos (249MB/s), which are more similar.
  • Sostenibility: The Exos disks are much more robust (more workload, MTBF and Warranty).

I'd say that for 104$ it makes sense to go with the IronWolf Pro + Exos 7E8 combination.

Choosing the disks for the cache

Using a ZLOG greatly improves the writing speed, equally using an SSD disk for the L2ARC cache improves the read speeds and improves the health of the rotational disks.

The best M.2 NVMe SSD for NAS caching are the ones that have enough capacity to actually make a difference to overall system performance. It also requires a good endurance rating for better reliability and longer lifespan, and you should look for a drive with a specific NAND technology if possible.


"If you want a TL;DR you can jump to the conclusion."

To choose your disks take into account:

Cache disk NAND technology

Not all flash-based storage drives are the same. NAND flash cells are usually categorised based on the number of bits that can be stored per cell. Watch out for the following terms when shopping around for an SSD:

  • Single-Level Cell (SLC): one bit per cell.
  • Multi-Level Cell (MLC): two bits per cell.
  • Triple-Level Cell (TLC): three bits per cell.
  • Quad-Level Cell (QLC): four bits per cell.

When looking for the best M.2 NVMe SSD for NAS data caching, it’s important to bear the NAND technology in mind.

SLC is the best technology for SSDs that will be used for NAS caching. This does mean you’re paying out more per GB and won’t be able to select high-capacity drives, but reliability and the protection of stored data is the most important factor here.

Another benefit of SLC is the lower impact of write amplification, which can quickly creep up and chomp through a drive’s DWPD endurance rating. It’s important to configure an SSD for caching correctly too regardless of which technology you pick.

Doing so will lessen the likelihood of losing data through a drive hanging and causing the system to crash. Anything stored on the cache drive that has yet to be written to the main drive array would be lost. This is mostly a reported issue for NVMe drives, as opposed to SATA.


DWPD stands for drive writes per day. This is often used as a measurement of a drive’s endurance. The higher this number, the more writes the drive can perform on a daily basis, as is rated by the manufacturer. For caching, especially which involves writing data, you’ll want to aim for as high a DWPD rating as possible.

Cache disk conclusion

Overall, I’d recommend the Western Digital Red SN700, which has a good 1 DWPD endurance rating, is available in sizes up to 4TB, and is using SLC NAND technology, which is great for enhancing reliability through heavy caching workloads. A close second place goes to the Seagate IronWolf 525, which has similar specifications to the SN700 but utilizes TLC.

Disk Size Speed Endurance Warranty Tech Price
WD Red SN700 500 GB 3430MB/s 1 DWPD 5 years SLC 73$
SG IronWolf 525 500 GB 5000MB/s 0.8 DWPD 5 years TLC ?
WD Red SN700 1 TB 3430MB/s 1 DWPD 5 years SLC 127$
SG IronWolf 525 1 TB 5000MB/s 0.8 DWPD 5 years TLC ?

Choosing the cold spare disks

It's good to think how much time you want to have your raids to be inconsistent once a drive has failed.

In my case, for the data I want to restore the raid as soon as I can, therefore I'll buy another rotational disk. For the SSDs I have more confidence that they won't break so I don't feel like having a spare one.


Mount a pool as readonly

zpool import -o readonly=on {{ pool_name }}

Mount a ZFS snapshot in a directory as readonly

mount -t zfs {{ pool_name }}/{{ snapshot_name }} {{ mount_path }} -o ro

List volumes

zpool list

List snapshots

zfs list -t snapshot

Get read and write stats from pool

zpool iostat {{ pool_name }} {{ refresh_time_in_seconds }}


Last update: 2022-10-14