What is CMR?
CMR stands for “conventional magnetic recording” and refers to the traditional way that data is written to magnetic hard disk drives. In CMR drives, the tracks where data is stored are separate from each other with space in between. This allows for predictable write performance since writes do not affect adjacent tracks.
With CMR, each track is written contiguously in concentric circles across the disk platter. The head can write to any available location on the track without affecting other tracks. This makes CMR well-suited for random writes and rewrites.
Some key benefits of CMR drives include:
- Higher performance – CMR enables faster random writes compared to SMR.
- More reliable – Separate tracks prevent write errors and data loss issues.
- Stable latency – Consistent performance over the lifetime of the drive.
- Backwards compatible – CMR works with all existing HDD interfaces and uses.
In summary, CMR is the conventional way hard drives have stored data for decades. The isolated tracks provide predictable performance and reliability. CMR remains the technology of choice for applications requiring high write performance and random access writes.
History of CMR
CMR (Conventional Magnetic Recording) technology was first developed in the 1950s. According to a Reddit discussion on the history of SMR drives, CMR allowed data tracks to be stored individually without overlapping on hard disk platters, enabling higher densities than previous longitudinal recording methods [1].
Some key innovations in CMR technology over the years include the introduction of thin-film heads in 1979, which allowed for narrower tracks and greater storage densities. Later, IBM introduced the first commercial use of magnetoresistive (MR) heads in 1991, further boosting areal densities. Giant magnetoresistive (GMR) heads were introduced by IBM in 1997, providing even greater sensitivity for reading data.
Over decades of incremental advancements, CMR technology achieved major leaps in hard drive storage capacities. Whereas early disk drives in the 1950s-60s measured storage in megabytes, CMR enabled capacities to reach into the terabytes by the 2000s.
How CMR Works
CMR drives use magnetic recording principles to write data to and read data from tracks on a platter. Writing data involves orienting magnetic material on the platter to represent 0s and 1s. The drive’s write head generates a magnetic field that aligns magnetic material in the desired orientation as the platter spins. To read this stored data, the drive’s read head detects the magnetic orientation as it passes over the platter. The changes in magnetic flux generate an electrical signal that is decoded into 1s and 0s.
CMR drives write tracks sequentially and read/write heads can access any track independently. This allows for reliable performance and the flexibility to access data in any order. Each track is written in continuous concentric rings across the platter with thin guard bands between tracks. This maximizes the capacity while keeping cross-talk interference between tracks low.
By comparison, shingled magnetic recording (SMR) drives partially overlap tracks to increase platter density. This requires tracks to be written sequentially and can limit performance in some scenarios.
CMR vs SMR
CMR (Conventional Magnetic Recording) and SMR (Shingled Magnetic Recording) are two different hard drive technologies that differ in how data is written to the drive platters.
CMR writes data in separated, parallel tracks on the drive platters, while SMR overlaps the tracks in a shingled pattern to achieve higher data density. This key difference affects performance and use cases for each technology.
Some key differences between CMR and SMR drives:
- Performance – CMR allows for faster write speeds and overall better performance as data can be directly accessed. SMR has slower write speeds due to needing to rewrite overlapping tracks.
- Overwrites – CMR can directly overwrite existing data. SMR requires rewriting entire sections to overwrite data due to the overlapping tracks.
- Use cases – CMR is better for applications requiring fast write speeds like RAID arrays. SMR is more suitable for sequential writes like archival data storage.
In summary, CMR is generally faster for both reads and writes while SMR fits applications where capacity is more important than performance. For uses like servers and NAS devices, CMR is usually preferred for its better overall speed and random access capabilities.
Sources:
https://www.howtogeek.com/803276/cmr-vs.-smr-hard-drives-whats-the-difference/
Uses of CMR
CMR technology is commonly used in numerous storage devices and applications where performance and reliability are critical. Due to its design, CMR excels in high performance scenarios such as:
- Desktop and laptop hard drives
- Enterprise server drives
- High-end NAS devices
- External USB hard drives
- Gaming consoles like Xbox and PlayStation
CMR drives are the technology of choice for operating systems and applications where data access speeds need to be consistently fast. The sequential writing method allows CMR drives to deliver solid performance when reading and writing large blocks of data. This makes them well-suited for tasks like:
- Running demanding applications like video editing software
- Recording and retrieving large media files
- Handling complex data operations on servers
- Storing games without long load times
Overall, any use case where consistent, fast drive performance is required tends to benefit from CMR technology. The design provides predictable speeds necessary for primary storage in PCs, servers, high-performance NAS, and other devices.
CMR Reliability
CMR (Conventional Magnetic Recording) hard drives offer improved reliability and data integrity compared to SMR drives. The non-overlapping tracks used in CMR drives reduce the chance of data corruption and loss. Studies have shown CMR drives have a lower annualized failure rate than SMR drives.
CMR drives provide better long-term reliability as well. The shingled writing method used in SMR drives eventually leads to read/write issues as tracks become more densely packed. CMR drives do not have these same physical limitations, allowing them to withstand heavier workloads and last longer before failure.
For applications requiring high data integrity like financial records, media storage, or gaming, CMR drives are the better choice. According to experts 1, the more robust reliability of CMR makes them preferable for write-intensive or mission critical workloads.
CMR Performance
CMR hard drives are known for their excellent sequential read and write speeds. Unlike SMR drives, CMR drives can sustain high sequential transfer rates because there is no read-modify-write cycle required. According to tests done by StorageReview, CMR drives achieve over 200 MB/s sustained sequential read and write speeds.
In addition to great sequential speeds, CMR drives excel at random access performance. Since the tracks are discrete and non-overlapping, the heads can move rapidly between tracks to access data located anywhere on the platter. Typical random access times for CMR drives are in the 10-15 millisecond range.
The combination of high sequential transfer rates and fast random access gives CMR drives very low overall latency. For example, an average CMR HDD has a latency around 5-10ms for reads and 10-20ms for writes. This low latency makes CMR drives well-suited for applications that require both streaming speed and responsiveness.
CMR Capacities
Typical CMR drive capacities range from 500GB to 20TB for 3.5″ desktop drives and up to 10TB for 2.5″ notebook drives. CMR technology allows for high capacities thanks to its conventional recording method that writes tracks consecutively without overlap.
However, there are physical limitations to how densely tracks can be packed, which restricts maximum capacities for CMR drives. As tracks get squeezed closer together, interference and crosstalk between neighboring tracks increases. This limits areal density and makes further capacity growth challenging for CMR.
For example, Seagate’s 20TB CMR drive uses nine platters to achieve the maximum capacity while avoiding excessive track misregistration. Fitting more platters is difficult for smaller form factors like 2.5″ drives, limiting their maximum CMR capacities compared to 3.5″ drives.
To continue growing capacities in the same HDD form factors, technologies like SMR and HAMR are required to push drive areal density higher. But for now, CMR provides a good balance of high capacity and performance up to 20TB for desktop drives.
Shingled Magnetic Recording (SMR)
Shingled magnetic recording (SMR) is a newer hard drive technology that was developed to enable higher storage densities than conventional magnetic recording (CMR). In SMR, the data tracks on the disk partially overlap like shingles on a roof, allowing more tracks to fit onto the disk surface. This increases overall storage capacity compared to CMR.
The key difference between SMR and CMR is in how data is written. With CMR, tracks are written independently so any track can be updated at any time. SMR requires writing data sequentially and updating existing data involves rewriting larger sections. This makes SMR drives better suited for sequential write workloads like archiving rather than random write usage.
While SMR enables higher capacities, it has downsides like lower performance for random writes and potential data integrity issues if used outside its intended workloads. SMR is less reliable than CMR and is not recommended for mission critical applications that require high performance and availability.
Many hard drive manufacturers have introduced SMR drives but they are often not clearly labeled as such. Some NAS and enterprise drive models have quietly transitioned from CMR to SMR, creating issues if used in workloads unsuited for SMR.1 Consumers should research drives carefully to ensure SMR vs CMR compatibility with intended usage.
The Future of CMR
CMR hard drives have been the dominant hard drive technology for decades, but emerging innovations may change how data is stored in the future. Some key developments that could impact CMR drives include:
Heat-assisted magnetic recording (HAMR) uses lasers to heat up high-capacity disk areas to make them easier to write data to. Seagate expects to release 20TB+ HAMR drives soon, allowing much greater capacities than current CMR drives [1].
Microwave-assisted magnetic recording (MAMR) is another technology that aims to push drive capacities higher by using microwaves to make the media easier to write to. Western Digital aims to release 50TB MAMR drives by 2026 [2].
Bit patterned media could potentially increase capacities 10x further by physically organizing magnetic regions in a regular pattern. However, it faces challenges with economical manufacturing [2].
CMR drives may also find new applications in cold storage, archival storage, and personal storage as capacities increase. High-capacity drives reduce costs for rarely accessed “cold” data. CMR’s random access abilities still provide advantages over tape archiving. And consumers continue to need high-capacity drives for their exploding digital media libraries.
