In recent weeks there has been significant controversy caused by reports that a number of big hardware vendors are quietly shipping SMR disks in their NAS (Network Attached Storage) products.

The reasons for this are not particularly surprising. By using SMR, vendors can, as this Ars Technica article does a good job of explaining, “eke out higher storage densities, netting more TB capacity on the same number of platters — or fewer platters, for the same amount of TB.” This is at the cost of performance, as well as a number of other compatibility issues.

While the companies involved have responded to criticisms from both users and sections of the tech press, we thought the controversy would be a good opportunity to take a closer look at current trends and issues across the spinning disk market, including SMR, TRIM, and RAID.

There’s undoubtedly a lot of technical complexity at play here. This makes it particularly hard to determine the extent to which the companies alleged to have been using SMR were acting with malicious intent or merely failed to properly anticipate product issues for users. However, because such hardware can have a significant impact on performance and system resilience, understanding the differences between them is absolutely essential. For this reason, we believe manufacturers should have been clear from the start which devices utilised SMR.

What is SMR (Shingled Magnetic Recording)?

Innovation in magnetic disk technology is always moving towards higher data density and lower cost per GB. SMR continues this trend. However, unlike other recent incremental improvements, it has a significant impact on device performance.

A traditional disk with standard, non-overlapping tracks is typically called a CMR (or sometimes PMR) disk. Each CMR track (and a sector within a track) can be written without overwriting neighbouring tracks; the ‘cost’ of an update is only the time to slew the disk head and the sector to rotate under the head.

SMR uses the property that a narrower track can be read than the minimum written width. By writing overlapped tracks, the achievable data density can be significantly increased. This increase in density should in principle reduce the cost per GB and a small power reduction per GB where the number of platters can be reduced.

There is a cost, however; to write an update, all the tracks have to be re-written. The primary mitigation is to divide the shingled areas into zones. This limits the re-write to the extent of the zone, instead of the entire disk surface.

To update a byte on the disk, the entire zone must be read, updated in memory, and then written back out to disk. This is known as write amplification. Flash media is similarly impacted, although to a lesser extent. Due to its structure, erasure must be applied to a whole page of memory.

Read more. Source: Macrium’s view on the recent SMR disk controversy

Posted By: sygnus21
30 Apr 2020