What are SMR HDDs (and why you should avoid using them in a NAS)
Unlike SSDs, which can have radically different form factors and interface options, HDDs don’t have that many distinctions. Aside from the size difference between 2.5 and 3.5-inch hard drives, RPM is often the most important statistic that sets two hard drives apart. Or so you might think, until you encounter SMR and CMR variants of the same HDD.
While they may sound similar, there’s a world of difference between the two. I wouldn’t necessarily say that the former are inferior in every way, but there are enough reasons why you should avoid using SMR drives on your NAS setup.
What are SMR drives?
And what’s special about them?
Instead of writing data on discrete sectors, drives based on the Shingled Magnetic Recording (SMR) technique save your files inside semi-overlapping tracks. Similar to their namesake, data inside SMR drives is stored in a fashion identical to the arrangement of roof shingles: each shingle (or record, in the case of the HDD) overlaps with its successive shingles (records). This way, SMR drives can achieve higher storage density than their CMR rivals.
Since SMR hard drives can store more data on fewer platters, they are cheaper to produce, and often have a lower cost per terabyte than their mainstream counterparts. Combine their higher storage density with the fact that they’re a fairly new technology, one might wonder if they’re better than CMR HDDs. Well, the answer to that is a big no.
Why should you not use SMR drives?
You'd really want to avoid them for NAS workloads involving frequent write operations
The biggest disadvantage of SMR drives is their poor write speeds. Going back to the analogy between SMR drives and shingled roofs, changing a specific shingle is a bit difficult due to the overlapping aspect of these roofs. Similarly, since the tracks in SMR drives have an overlap, their write speed becomes painfully slow when you need to overwrite data.
Therefore, you don’t want to run an SMR drive with a storage server that has constant write operations. If you’re running a conventional RAID setup, you should stay away from these HDDs. Due to their slow write times, they’ll end up tanking the transfer speeds when you send any files to your RAID-powered NAS.
Consider a hypothetical scenario: One of your drives unexpectedly bit the dust, and you need to replace it with a new one. When you rebuild your storage pools after slotting in the SMR drive(s), the resilvering process will proceed at a snail’s pace because of the way these HDDs operate. If you’re as much of a data hoarder as I am, your server might take days to finish the rebuilding process.
When should you use SMR HDDs?
Only when you’re absolutely sure you won’t need to overwrite data
While I’d always recommend going for CMR drives if you value your time (and sanity), SMR hard drives serve as decent stand-ins for Write Once, Read Many (WORM) tasks. If you’re planning to store media files for archival purposes, SMR HDDs are worthwhile thanks to their low prices.
