Wikipedia explains all:
[snip]
All current drives you'll buy will use either PMR or SMR, unless you are really into some exotic, super-dense magnetic disks. PMR has been the standard for a very long time, and disks might not even say they use it because it is so standard, and has no performance implications
I think I'm getting it with the help of you guys. I've read the definitions, it just sounded to me like SMR was how it was laid out on the platters, as opposed to the technological means of encoding.
I guess another part I was missing is the brands aren't shy about crowing about using these fancy technologies they poured money into for years, but it was still a mystery if they were using SMR or not. If you see what I mean? "Yes, we laser heat our drives, but we aren't going to tell you if we use SMR!"
Actually, I just realized my main source on the topic says SMR uses PMR, and CMR is 'pure' PMR. It's all a bit confusing. It really sounds like it's a layout vs encoding tech topologies kind of thing. "Just when I thought I was out they pulled me back in!"
This was one of my main references, translated & paragraphs rearranged by topic in my notes for clarity for my own uses (apologies for any mangling):
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www.hardwareluxx.de
Since PMR is also used within SMR technology, one usually speaks of CMR, conventional magnetic recording, when hard disks with a pure PMR recording process are meant. If you are aware of these two technologies, you can make good use of advantages and disadvantages.
Manufacturer Seagate used, for example, SMR (device managed) for the hard disks of the BarraCuda (3.5 inch), FireCuda SSHD (2.5 inch) and the BarraCuda and BarraCuda Pro drives in 2.5 inch format . The BarraCuda Pro 3.5-inch drives and all models in the IronWolf, IronWolf Pro, SkyHawk AI and Exos X series are CMR drives. Both versions of the SkyHawk series are either already on the market or announced as SMR. In normal marketing, no further reference is made to the special features of the respective recording technologies. Fortunately, however, Seagate offers extensive data sheets and operating instructions that make the necessary information quickly accessible to interested consumers.
Manufacturers Toshiba and Seagate have been using the PMR process since the middle of the last decade. This method became necessary to provide more than 750 GB in conventional 3.5 inch hard drives and in SAS hard drives hard drives
At Western Digital, only certain series of the UltraStar series (formerly HGST) are currently marked with SMR recording technology (device managed, as well as host-managed). The new 2018 Red and Blue models also come with cache features (see our article on the WD60EFAX), or certain Red models are already confirmed SMR hard drives. In contrast to Seagate, there is no explanation of the recording method here. The same applies to the manufacturer Toshiba, who confirmed on request that no information on the recording method used can be published. Meanwhile, it is also known here that models of the P300 series use SMR without being marked as such.
Regrettably, the manufacturers WD and Toshiba hardly or not at all communicate with the customer. The manufacturer Western Digital continues to advertise its RED hard drives with the slogan "Developed for use in NAS".
With MAMR hard drives from WD and HAMR hard drives from Seagate, innovative recording methods are coming in the near future, with which even more capacity can be accommodated in the 3.5-inch housing. The first 20 TB hard drive (SMR) should be available later this year.
If the manufacturer doesn't say anything, it's up to the user to determine what they bought. However, not all SMR drives behave the same, so identification is even more difficult. The general rule is: If an SMR hard disk is written to beyond the capacity of its buffer, the transfer rate is usually reduced by half. Using the example of different hard drives, we will show what you can also pay attention to. If you are unsure about the interpretation of the HD Tune graphs and CDM values, you can use the Seagate IronWolf Pro 16 TB ST16000NE000 (our article) to get an idea of what events a pure CMR/PMR hard drive delivers.
In comparison, we take a model from the same manufacturer and the same era. According to the data sheet, the Seagate IronWolf 6 TB ST6000VN0033 has no SMR, but a kTpi of 370 and thus uses a conventional recording method. Here we see that the disk can write without interruptions.
We also noticed the sawtooth we described when writing in HD-Tune in another Seagate model with confirmed SMR.
We summarize what are the key points we have seen from confirmed SMR hard drives.
- non-continuous writing
- writing high 4K values
In addition, there may be the conspicuously implausible reading with HD-Tune.
We also dealt with traditional writing activities. Because of the way it works, a drive with SMR must theoretically perform worse here than a conventionally written drive. To what extent the write rate is significantly lower and whether such drives are really useless, as some people think, we will clarify here. Of course, read and write tests do not cover all possible uses, but hard drives have now been replaced by SSDs as games or program drives. We decided to use the Windows display method because HD-Tune's recording function displays the result logarithmically. The curves are easy for everyone to interpret and the exact transfer rate doesn't
play that big a role in this test.
WD SMR drives use Full LBA Indirection. Unlike CMR drives, SMR drives allow the LBA's mapping to physical location to change with each write. This requires HDD resources to keep track of LBAs that are mapped to a physical location (translation table like what is used for SSDs).
Trimming releases those resources and frees up the physical media space to be reused. Trimming also relieves the drive of background activities associated with the 'un-needed' write data.
Only the host can tell the drive what data is no longer needed. The drive will never delete user data on its own. A Trim command will tell the command what specific data is no longer needed.
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