Differences between SMR, CMR, LMR and PMR hard disk: does it have something to do with Linux?

Hard disk, differences CMR, SMR, PMR

Well, the quick answer to the title is no. But that may not be the case at all, but we must start at the beginning. And is that these terms LMR, SMR, CMR and PMR you may hear them more and more. Especially if you are looking to buy a magnetic hard drive (HDD) and you are trying to inquire about the technologies to choose a good one.

Surely you have seen that lately there is a lot of talk about SMR technology for modern hard drives. For example, Wester Digital, or WD, has recently launched the Red Plus and Red Pro lines that are exclusively CMR, and has also had to come out to disprove problems with its SMR units. But what are all these acronyms? What differences are there? Are they really related to Linux or not? I will try to clarify all these questions ...

Differences between LMR, CMR, PMR and SMR

Platters and hard drive head

Headstock & Chainrings: Seagate Medalist ST33232A

The first thing you should know is that HDD hard drives, that is, magnetic or mechanical ones, use magnetism as a medium for writing and reading the data on the surface of the discs.

Without going into too much detail about the composition of the dishes, and other details, I go directly to differentiate the ways in which these memory accesses are made. That is, the types of MRs (Magnetic Recording) that exist:

  • Longitudinal (MRL): It is a type of data storage where it is stored longitudinally on the surface of the disk. The hard disk head will be able to magnetize the area in one way or another (north-south) in order to create ones and zeros for the binary information. It is the classic way how information was stored on older hard drives.
  • Perpendicular (PMR): Seagate was one of the first to use this technology for hard drives from 750 GB capacity. It had a clear advantage over the LMR since, being perpendicular, each data took up less space and more information could be stored on the same disk surface. In addition, it heats up less by retaining the info in more regular and stable areas.
  • Conventional (CMR)- The rest of the manufacturers also started using PMR for their hard drives, which is why it ended up becoming the norm in this hard drive industry. That is why it was called CMR as it was already widespread and conventional. But it is the same as PMR.
  • Shingled (SMR): with the incessant struggle to achieve higher data density per square centimeter, to be able to make hard drives with more and more capacity with the same number of plates and size, SMR technology was also created. A type of recording that differs from the previous ones by being staggered. In this type of technology a reader head is used that is smaller than the writing head, and the data tracks are superimposed on each other. This increases the possibility of recording more data in the same area unit, that is, the density increases. The problem is that it can happen that a track is overwritten when trying to delete or modify a stored data, which would lead to data corruption. The way to solve this problem is to write all the data that has to be modified in a separate sector and when there are downtimes of the use of the hard disk, it takes care of reordering the data. Something similar to what happens in SSDs with TRIM and over-provisioning (overprovissioning). But that has problems, since several writes really have to be done when with other technologies only 1 should be done ... therefore, the increase in density in this case has a cost in terms of writing penalties.

In short, in the latest hard drives that are being sold, whatever the brand they are, you can find yourself CMR or SMR. For example:

  • Seagate- Newer Barracudas from 1TB to 8TB are usually SMR. While the Ironwolf are usually CMR.
  • Toshiba- Many of their 1TB to 6TB drives are usually SMR. Others like the X300, P300 and N300 are usually CMR.
  • Western Digital: it has a very heterogeneous variety, with Red series that mix SMR and CMR. The Red Pro are CMR, Blue mix, Black are CMR mostly with a few exceptions, and Purple are CMR.

And what does it have to do with Linux?

raid, Linux storage server

Well, the first thing is that Linux is present in most of the servers, and also in many supercomputers. And these use settings of RAID storage. Redundant systems "don't get along very well" with SMR. At the very least, they should know if they have SMR hard drives or if they are mixed with other types of hard drives. Otherwise, they could cause serious problems.

Note that with RAID it is used writing simultaneously in several units at the same time. For example, in a RAID 1 (mirror or mirror), everything that is written to a hard disk A is also written to a B in order to have an exact copy of the data and in case of failure of one of the drives, have another backup ...

Our alterations in SMR could make these disks take too long to write data versus using CMR-only RAID systems. However, there are RAID systems where all their drives are SMRs and there is not too much of a problem, but it is important that technical personnel dedicated to replacing drives in a RAID system are aware of this.

There are practical cases, such as Dropbox cloud storage servers where nodes with SSDs and nodes with HDD SMR are used. But there is a trick, they are not together, but the SSDs are used as a buffer or cache to accelerate the speed and when they have 1GB they go on to write it in 4 blocks of 256MB of the HDDs. Therefore, they complement each other, but they do not mix ...

In fact, some people who have bought hard drives for NAS with RAID configuration and the new drive was SMR, saw problems appear marking the drives as "degraded" or saw how rebuilding took longer than usual when replacing one HDD unit with another.

But apart from the RAID system, there is another big problem for SMR, and it's the XFS file system, also widely used in Linux environments. XFS is widely used in NAS, and it causes that every time you want to rewrite a 4KB sector it implies reading and rewriting the entire 256 MB. That makes the transfer fees totally dire.

Conclusion, for this type of RAID technologies you should avoid mixing SMR with CMR and for NAS you should avoid using XFS. But personally, I recommend you opt for the CMR and thus avoid limitations and headaches ...


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