The factors of writing and erasing speed on SMR hard disks
The background of HDDs and SMR hard drives:
Mechanical hard disk is composed of spindle motor, platters, read/write head, VCM actuator, actuator arm etc. The hard disks record data by rotating and moving actuator arm to change the electromagnetic flow and the polarity of magnetic recording particles of HDDs.
With the increasing amount of data, the demand for larger and lower-cost storage capacity is also increasing. SMR (Shingled magnetic recording) hard disk is first launched by Seagate in 2013, which is common for large-capacity HDDs above 2TB.
The appearance of SMR HDDs is same as general mechanical hard drives. Some hard drives are not marked it are SMR HDDs, which need to be checked on the official websites or customer service of the manufacturer of HDDs to confirm it is a SMR HDD or not.
How it works? SMR, PMR (CMR), LMR hard disks
To obtain a larger capacity per unit area, the permutation of magnetic recording particles on hard disks is constantly evolving from LMR (Longitudinal Magnetic Recording) to PMR (Perpendicular magnetic recording) known as CMR (Conventional magnetic recording), and now to SMR (Shingled magnetic recording).
(Figure 2) Schematic diagram of head and magnetic particles of LMR hard disks
(Figure 3) Schematic diagram of head and magnetic particles of PMR/CMR hard disks
Because the write head of the hard disk is larger than the read head, the read head does not need to occupy the entire track. SMR hard disk uses the way of partially overlapping the tracks, which can increase more capacity in the same unit area and reduce the cost per unit area.
(Figure 4) Schematic diagram of write head, read head and tracks (PMR hard disk)
(Figure 5) SMR hard disks use overlapping to obtain larger unit capacity, but it takes more time to rewrite data
SMR hard disks are suitable for cold data storage applications that do not require a lot of writing operations, such as surveillance storage, NAS backup, etc.
SMR hard disks features that when the data is rewritten, the original data needs to be moved to the blank space on the hard disk because the partially overlapping the tracks of SMR HDDs.
While writing new data, the hard disk will reorganize other tracks, which greatly reduces the writing speed, so it is not suitable for applications that require a lot of rewriting.
The factors of erasing speed slow down on SMR hard drives:
hard disks are consumable items. After the hard disks being using for a long time, to maintain the quality of data storage, the hard disk needs to be replaced.
The replaced hard disks should be destroyed or erased in compliance with the legal requirements of government laws for the protection of personal privacy.
The data-erasing way of hard disk is overwriting (0, 1 or random numbers) and erasing times on the hard disk.
When erasing the hard disk, which is an overwrite command to the hard disks. When erasing (overwriting) the SMR hard disks which full of data, the writing speed may drop significantly, which is a normal phenomenon.
(Figure 6) Schematic diagram of erase command to hard disks
With the expansion of scale of Data Centers, the importance of a single hard disk has been deemphasized. To pursuit lower unit storage costs in the application field of cold data storage, the market share of SMR hard disks has gradually increased.
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