SMR, CMR and PMR NAS Hard Drives- A Buyers Guide 2021

A Guide to CMR/PMR and SMR drives in 2021 – Should You Buy Them?

Remember when buying a hard drive was simple? The technology behind hard drives has existed for decades and it is only really in the last 10 or so years that the formula for how hard drives store data has changed substantially. Just over a year ago, Western Digital got into all manner of trouble with hard drive buyers when it was announced that a number of their popular range of NAS server hard drives featured a variant of shingled magnetic recording, (know more commonly as SMR). I say a version, as there are indeed several different ways that a hard drive can implement SMR architecture internally, with the two more common branches being Drive Managed (DM-SMR, which is what the WD Red drives in question use) and system-managed data allocation and coalition (SM-SMR). The suitability of SMR hard drives in NAS systems has always been questioned, due to the argued inferiority, inconsistency and performance versus CMR/PMR drives (Conventional Magnetic Recording and Perpendicular Magnetic Recording), hence the outcry when it was revealed that some WD Red hard drive models feature SMR. Although SMR hard drives have existed for quite a long time and the architecture itself has its uses, typically these drives are understood as singular use drives and not for large-scale RAID implementation. Today I want to explain a little about what SMR and CMR/PMR drives are, should you use them and list which drives use which magnetic recording method.

What are CMR/PMR Hard Drives and Should You Use Them in NAS?

CMR and PMR are effectively different names for the same kind of Drive based Magnetic Recording. A hard drive is made up of numerous circular platters and data is written on to these platters over time. The data is both read to and written from the drive, utilising an internal arm that moves across the disks to apply or retrieve data, with a larger indexing system being managed inside the drive controller and firmware. Data is read and written as and when required on the disc, with data aligning side by side in circles across each platter – hence the word perpendicular in the name. Occasionally, as data is deleted or refreshed on the drive, spaces appear across the platters as the original data was spread throughout the whole drive. This explains the need for occasional defragmentation that cleans up the recorded data to the disk into more manageable and tidy arrangements. Performance is largely dictated by the speed at which these platters rotate for the arm to read and a small area of memory used for live data handling, known as cache. CMR/PMR hard drives have been used consistently in the world of NAS servers for years thanks to the consistent and reliable performance in general access, but also when a RAID storage system requires recovery with a RAID rebuild, resync and re-silvering. Although the general technology and development of NAS hard drives has changed over the last decade (with more tailored architecture and firmware for 24×7 use), the bulk of NAS hard drives are CMR/PMR based.

What are SMR Hard Drives and Should You Use Them in NAS?

SMR was originally developed as a means of increasing the total available storage on a hard drive. In many ways, the architecture of a SMR drive is identical to that of CMR/PMR, but the big, BIG difference is in how data is written on the platters. The lines that data is written onto located on each platter is fractionally overlapped as write actions take place, much like the shingle tiles on a roof of a building (hence the name) and this allows a larger area of space to be made available on a hard drive compared with perpendicular/conventional write actions. The drive generally needs more cache in order to do this, which in turn can often lead to improved performance on these drives – something that some brands will make a point of highlighting. However, this shingled layout of data is exactly why these drives are less desirable for NAS server use. In most cases, the drive controller will require more time to sort the layout of data on the platters as data is written, often done when the drive is idle. However, when a drive is being constantly accessed, it results in the drive having no time to reorganize the written data as the disc steadily fills. Additionally, in the event of a drive failure, during RAID rebuilding, an SMR Drive will likely take considerably longer to repair on particularly full arrays and often the required idle time for data arrangement is not possible, likely leading to RAID rebuild and RAID resync issues and potentially lost data. This is the main crux of the argument of most people against the use of SMR drives in a NAS server.

Other Hard Drive Types in NAS? What Are HAMR and EAMR Hard Drives?

It is also worth highlighting that in 2020/2021 a couple of new hard drive magnetic recording methods have begun to appear on the market in efforts to achieve even larger storage capacities, while still maintaining performance and reliable storage. Different hard drive brands have approached the subject of using specific area targeted effects on the storage platters in efforts to expand the available area, allowing greater density of storage. Seagate has utilised HAMR, known as heat-assisted magnetic recording. Whereas WD is utilising energy-assisted magnetic recording, or EAMR. Both methods are based on utilising increased temperature in modifying platter write space, as well as taking advantage of helium drive seal technology and for the purposes of this guide, both can be generally considered comparable to PMR/CMR hard drives and not SMR.

Additionally, Seagate is working on drives featuring dual-arm/actuator controlled drives that effectively double read-write performance with their Mac2 series, but these are not quite commercially available yet.

Recommended Hard Drives for NAS?

If you are still wondering about the best Hard Drive to use in your NAS system, as well as wondering the pros and cons of each brand (Speed, Price, Noise, Endurance and more), use the guide below to go through each of the brands and their ranges to see which one is best for you.

Click below to head over to the NAS Hard Drive Help Guide (Will open in a new window)

 

SMR and CMR/PMR NAS Hard Drives – Which One is Which

The following list of hard drives has been broken into drives that are SMR and drives that are CMR/PMR. Take special note of which drives have suitability for small/big NAS enclosures, as this will make a great deal of difference towards their suitability in larger RAID systems. Remember, Western Digital still include a number of SMR drives in there are WD Red range which is Drive Managed SMR (DM-SMR) and still certifies that these can be utilised in smaller NAS systems. For those that are concerned about their suitability, however, you can still use the WD Red Plus series, which is exclusively CMR/PMR in architecture. Alternatively, the Seagate ironwolf range arrives exclusively in CMR/PMR, with no SMR drives present in their NAS server series.

WD Red (Standard Class) – SMR Hard Drives

Below is the Currently available range of WD Red Hard Drives that arrive in SMR form.

 

WD Red Plus (Standard Class) – CMR/PMR Hard Drives

Below is the Currently available range of WD Red Plus Hard Drives that arrive in CMR & PMR form.

 

Seagate Ironwolf (Standard Class) – CMR/PMR Hard Drives

Below is the Currently available range of Seagate Ironwolf NAS Hard Drives that arrive in CMR & PMR form.

 

Seagate Ironwolf Pro (Business Class) – CMR/PMR Hard Drives

Below is the Currently available range of Seagate Ironwolf Pro NAS Hard Drives that arrive in CMR & PMR form.

Toshiba Hard Drives (Endurance Class) – CMR/PMR Hard Drives

Below is the Currently available range of Toshiba N300 NAS Hard Drives that arrive in CMR & PMR form.

 

Synology Hard Drives (Endurance Class) – CMR/PMR Hard Drives

Below is the Currently available range of Synology HAT5300 NAS Hard Drives that arrive in CMR & PMR form.

Are SMR drives unsuitable for a NAS?

In the short term and in a small NAS array, SMR hard drives are probably ok. You should probably still avoid them where possible, but in less intensive and non-business class use, they should be ok. The key thing to remember is that these drives constantly need time to reposition data internally on the platters and larger arrays, as well as more constant write activity servers, will not play ball in a way that these hard drives can support. The main reason that Western Digital and their WD Red series got in so much trouble was not that they used SMR class drives, it is because they used SMR class drives in their NAS series and did not make this clear enough to buyers.

Western Digital has tried to resolve this problematic PR miss-step with the introduction of the WD Red Plus series to bring a little more clarity to the WD Red portfolio, but for many users this is is still not quite enough contrition from the brand and has likely been quite a humbling instance for them, especially given their market share vs Seagate until recently. In summary, if you are using a 1/2-Bay NAS for private home use (low-level data, media, etc), I think DM=SMR hard drives should be ok. However in almost every other instance, big or small, I will always recommend PMR/CMR hard drive media, due to its more consistent RAID performance and more reliable RAID build, rebuild and resync when the situation requires it.

Learn More About the WD Red HDD DM-SMR Coverage Last Year and Performance Testing Below:

 

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