Last night, a Western Digital executive reached out to Ars to let us know of a blog post concerning their controversial Red drives.
The company is taking a new branding initiative to clarify the technology used in its NAS drives—in the near future, "WD Red" will exclusively mean disks using Shingled Magnetic Recording technology, and "WD Red Plus" will mean disks using Conventional Magnetic Recording.
Overview
This report is the latest in a series on hard drive manufacturers slipping SMR technology into existing disk lines, with little or no notice to customers.
In (very) brief, SMR disks generally perform well enough in light storage workloads, with plenty of idle time between storage requests—but they can fall catastrophically flat on their faces when hit with more demanding workloads. The ZFS filesystem, in particular, tends to present SMR disks with challenges they have difficulty handling.
Although all three remaining major hard drive vendors—Western Digital, Toshiba, and Seagate—have "submarined" SMR disks into existing channels without doing much to notify customers about it, only Western Digital has done so with disks designed specifically for NAS, or Network Attached Storage, use.
Unfortunately for Western Digital, NAS users tend to be significantly more technical than general consumers—and they frequently hit their disks with far more difficult workloads than Western Digital apparently tested or planned for.
From Red to Red Plus
The use of SMR technology in Western Digital Red is not going away—but moving forward, "Red" will exclusively mean SMR disks. The existing SMR models—WD 20/30/40/60 EFAX—will retain their existing model numbers and will retain their existing WD Red branding. Meanwhile, the CMR disks formerly known as WD Red—in sizes from 1TB to 14TB—get a new "WD Red Plus" branding and label, although their model numbers also remain the same.
Western Digital's new marketing for the SMR-equipped Red drives labels them as for SOHO use only and clarifies that this means low-intensity operations with lots of idle time in between—and no ZFS. For small business, "intensive," or ZFS workloads, there's the Red Plus line—which effectively just means the older, pre-SMR models for now.
There's also a Red Pro line targeted to maximum-performance applications. This line is unchanged—it existed with the same branding before the SMR fiasco, and it still exists with the same branding and same models today.
Understanding SMR limitations
There's a fairly large CMR cache area on the otherwise SMR disks, as well as a 256MiB volatile (RAM) cache. Western Digital doesn't publish details about the CMR cache area in its product datasheets, unfortunately—but typical estimates, such as what was shown in this blocksandfiles interview, range from 1GiB to 100GiB, depending in part on the size of the disk itself.
The CMR cache area on the drive will perform the same as an entirely CMR disk does—and if it has "downtime" between storage requests, the drive's firmware can spend that time reading data from the CMR cache and trickling it down to the considerably more limited main SMR storage area. Once permanently stored in the SMR zones, the data can be read at about the same speed as it would be from an equivalent CMR disk—SMR's performance limitations are strictly bound to writes.
If you never committed enough write operations at once to overflow the large CMR cache area, and you gave the drive extended amounts of idle time to "breathe," you'd see no performance difference between CMR and SMR disks—although, as some NAS users have commented, "you can hear them running all the time." This refers to the garbage collection process migrating data from CMR cache to SMR zones occurring in long idle times between operations.
Even if you commit enough large write operations at once to overwhelm the CMR cache, in our testing Western Digital's SMR firmware generally does a surprisingly good job of committing writes directly to the SMR zones. This good job is predicated on those writes being large, contiguous writes, however—and on them being new writes, not rewrites of existing data. In order to modify a single 4KiB sector in a 256MiB SMR zone, the firmware must read in the entire 256MiB zone, then modify that one sector, then write the entire 256MiB zone back out again.
Is the new branding enough?
The new branding is certainly a big step forward for more knowledgeable consumers who already know they don't want SMR—the only thing they need to know is "Red" means SMR, and "Red Plus" and "Red PRead More – Source
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