SMR (Shingled Magnetic Recording) and CMR (Conventional Magnetic Recording) are two different hard drive technologies that store data in different ways on the disk platter. SMR technology allows the data tracks to partially overlap like shingles on a roof, while CMR technology keeps the data tracks separate from each other. Both technologies offer certain benefits and drawbacks depending on the use case.
In this article, we will take a deeper look at how SMR and CMR technologies work, their key differences, benefits and drawbacks, and ideal use cases for each type of hard drive.
Definition of SMR
SMR (shingled magnetic recording) is a storage technology that increases storage density on hard disk drives (HDDs) by overlapping tracks on the disk like shingles on a roof. This allows more tracks to be written to each disk platter, increasing total capacity (Toshiba, 2022).
With SMR, the read/write heads partially overlap new tracks over previously written tracks. This overlapping (shingling) allows narrower track widths on disk, enabling more tracks per inch to boost capacity per platter by up to 25% over conventional HDDs (Toshiba, 2022).
However, overlapping tracks requires special writing techniques called read-modify-write. When new data overwrites a track, adjacent tracks must first be read, modified, then rewritten along with the new track to maintain data integrity. This makes random writes slower than conventional drives, while sequential writes can run at full speed (Toshiba, 2022).
Benefits of SMR
One of the key benefits of SMRs is their lower cost per terabyte of storage compared to CMR drives. SMR drives achieve a higher storage density by overlapping tracks, allowing each platter to store more data. This increased density results in a lower cost per terabyte (TB). According to Seagate, SMR drives can deliver up to 25% more capacity than equivalent CMR drives.
For example, a 10TB SMR drive may cost around $200, while a comparable 10TB CMR drive costs around $260. So the $/TB for the SMR drive is $20, while for the CMR drive it is $26. This makes SMR drives more affordable for high capacity, sequential write workloads like surveillance video storage and data archives.
SMR drives also have a smaller physical footprint thanks to their higher areal density. A 14TB SMR drive may have 3 or 4 platters, while a 14TB CMR drive usually has 7 platters. The smaller form factor saves space in storage servers and arrays.
Overall, the increased TB per dollar and TB per square inch are the two major benefits driving adoption of SMR drives for suitable workloads.
Sources:
https://blog.westerndigital.com/smr-cmr-choosing-right-storage-technology/
https://www.seagate.com/files/www-content/our-story/trends/files/idc-seagate-pmr-smr-brief.pdf
Drawbacks of SMR
SMR drives suffer from performance impacts compared to CMR drives, especially during write operations. This is because SMR drives write new data to the edge of the shingled tracks, which can partially overwrite adjacent tracks. To avoid data loss, the drive must first read the overlapping data into cache, rewrite the full track to a new location, then write the new data 1. This process is called read-modify-write and causes significantly increased latency and reduced throughput compared to standard drives. In addition, SMR drives exhibit write amplification, meaning more data must be written than the host requested to complete a write operation. This further reduces performance and endurance of SMR drives.
Definition of CMR
CMR stands for Conventional Magnetic Recording, which is a technique that has been used in hard drives for decades. With CMR, each bit is stored individually using the magnetic orientation of a defined area on the disk platter. The magnetic orientation of each bit cell can point in one of two directions to represent either a 1 or a 0. This allows data to be written sequentially in concentric circles on disk platters.
Each bit cell is separated by a physical boundary to prevent the magnetic fields from interfering with each other. This allows for reliable data storage and recovery. The individual bit cells are typically very small, allowing for high density data storage on disk platters. But the physical separation still takes up a minimum amount of space, limiting the total capacity using CMR technology.
CMR writes data in well-defined concentric tracks that do not overlap, making it easy to find and access the stored data later. This sequential recording method results in consistent performance when writing and reading data. However, it is not the most space-efficient technology as it requires physical separation between bit cells.
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Benefits of CMR
Caching Magnetoresistive RAM (CMR) offers better performance compared to Shingled Magnetic Recording (SMR) due to no rewrite amplification. CMR drives have higher random write performance since they don’t need to deal with the overlapping data tracks that SMR drives utilize to increase density. This avoids the performance penalty of needing to rewrite larger blocks of data sequentially that SMR drives face (https://cmrbenefitsgroup.com/). The lack of rewrite amplification is the main performance advantage of CMR.
Drawbacks of CMR
One of the main drawbacks of CMR is its higher cost per terabyte compared to other storage media like SMR. CMR drives have a more complex design and use higher quality components, which leads to a higher manufacturing cost that gets passed onto the consumer. This makes CMR drives less cost-effective for high capacity bulk storage use cases where low TB cost is a priority.
CMR drives also tend to have lower storage density than SMR. The simpler shingled writing approach used in SMR allows the tracks to be layered more densely, delivering higher capacity in the same physical space. The more complex CMR design limits how closely the tracks can be packed, resulting in lower total capacity for a drive of the same physical size as an SMR equivalent.
Sources:
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9030119/
Ideal Use Cases for SMR
SMRs are well-suited for applications that require reliable power with flexible siting, according to the U.S. Department of Energy (https://www.energy.gov/ne/benefits-small-modular-reactors-smrs). Some ideal use cases for SMRs include:
Archive and Backup Applications: The stable baseload power provided by SMRs makes them well-suited for archive and backup applications where continuous power is critical. SMRs can provide reliable power to data centers, archives, and facilities housing digital assets that require uninterrupted electricity.
Sequential Workloads: SMRs can efficiently support sequential workloads that involve steady power usage over extended periods of time. Applications like water desalination, hydrogen production, and district heating which need continuous stable energy input are well-matched to SMR capabilities.
Ideal Use Cases for CMR
CMR or Conventional Magnetic Recording media is best suited for primary storage with random workloads that require frequently updated data. This is because CMR uses perpendicular recording technology that allows for higher storage densities and faster write speeds compared to SMR. The random access performance of CMR makes it ideal for applications like:
- Operating system drives
- Application install drives
- General purpose enterprise server storage
- Desktop and notebook storage
- Gaming systems
- Point-of-sale storage
The higher cost per gigabyte of CMR is outweighed by its superior random write performance that is critical for primary storage workloads. Applications that require fast access to frequently updated data will benefit from the sustained write performance of CMR over SMR (https://www.snia.org/educational-library/shingled-magnetic-recording).
Conclusion
Overall, SMR hard drives are beneficial for read-intensive applications and storage with less frequent writes, whereas CMR hard drives are better for active storage and workloads with more frequent writes. The key differences include:
- CMR drives allow for individual magnetic tracks that do not overlap, enabling faster and more efficient writes. SMR drives have overlapping tracks, making writes more complex.
- SMR drives are cheaper per TB of storage, but have slower write speeds. CMR drives have faster writes but cost more.
- SMR drives are ideal for sequential data writes like archives, backups and NAS storage. CMR is preferred for transactional databases, enterprise applications, caching and OS drives.
- CMR drives have higher reliability and are less impacted by fragmentation over time. SMR drives can slow down as they fill up.
Overall, for most general consumer use cases like desktops and gaming, CMR drives are recommended for better performance. For large storage and backups that are mainly read, SMR offers a cost-effective option. Understanding the differences allows matching the right drive technology to the workload and use case.