What Is UltraSMR, and Are 26TB to 32TB SMR Drives OK for Servers?

Hard drives from 26TB to 32TB now come in two kinds that look almost identical on a quote: conventional magnetic recording (CMR) and shingled magnetic recording (SMR). The shingled versions get more capacity out of the same platters, and Western Digital markets its current approach as UltraSMR. Putting the wrong kind into a RAID array or NAS is an expensive mistake, because the drive will either not work at all or work badly when it matters. This guide explains the difference and which 26TB to 28TB drives fit which systems.

What is UltraSMR?

SMR writes each track so that it partially overlaps the previous one, like roof shingles. Narrower effective tracks mean more tracks per platter and more capacity per drive. The catch is that rewriting one track disturbs the tracks shingled over it, so data has to be written sequentially in large regions called zones. Changing a small piece of data in the middle of a zone means rewriting the zone.

UltraSMR is Western Digital's name for the SMR technology in its current host-managed Ultrastar drives. It combines shingled recording with additional encoding and error-correction techniques to add capacity beyond the conventional drives built on the same platform. For a buyer, the practical point is simple: UltraSMR drives are host-managed, so the extra capacity only comes with the software requirements below.

What is the difference between host-managed, drive-managed and host-aware SMR?

• Host-managed SMR: the drive exposes its zones to the operating system and rejects writes that break the sequential rule. The host must speak the ZAC (SATA) or ZBC (SAS) zoned command sets. The WD Ultrastar DC HC680 26TB, a 7200 RPM, 6Gb/s SATA, 512e drive with 512 MB cache, is listed in our catalog as host-managed SMR.
• Drive-managed SMR: the drive hides its zones behind an internal cache and remaps writes itself. It looks like a normal drive, but sustained random writes can fill the cache, and performance then drops sharply. This is the type behind well-publicized problems with some consumer NAS drives.
• Host-aware SMR: a hybrid that accepts random writes like drive-managed SMR but reports its zones so zone-aware software can optimize. It is rarely what enterprise buyers are offered.
• CMR: conventional, non-overlapping tracks. Any sector can be rewritten in place. Every RAID controller, NAS and mainstream operating system expects this behavior.

Are SMR hard drives OK for servers and NAS?

It depends on which SMR and which software. Host-managed SMR is fine, and cost-effective at scale, when the software stack was built for it: Linux zoned block device support, zone-aware file systems such as btrfs in zoned mode, or object storage and archive platforms that write large objects sequentially. Large archive and cloud-scale operators deploy it for exactly those workloads.

It is not fine for anything that assumes it can rewrite blocks in place. That covers hardware RAID controllers, standard Windows and VMware datastores, and NAS appliances. A host-managed drive in those systems is typically not usable at all. A drive-managed SMR drive will appear to work, then slow badly during heavy writes or a RAID rebuild, which is exactly when you can least afford it. Long, stalled rebuilds on drive-managed SMR are a well-documented problem in RAID and ZFS pools.

If the storage software was not written for zones, the drive should be CMR. At 26TB and above, that one check matters more than the brand on the label.

Which 28TB to 32TB drive should I buy?

Match the drive to the system it goes into. The 27TB and 28TB listings Nexus Compute carries split cleanly by use:

• RAID servers, JBODs and general-purpose storage: the Seagate Exos CMR 28TB (3WM103-886), a 7200RPM SATA enterprise drive explicitly listed as CMR. The Seagate Exos 28TB (ST28000NM000C) is also a 7200RPM SATA drive; confirm its recording technology on the product page before building a RAID set around it.
• Fast, verifiable drive retirement: the Seagate 28TB ISE drive (ST28000NM003K), 7200RPM on 6Gb/s SATA. ISE (Instant Secure Erase) lets you cryptographically erase the drive quickly when it is decommissioned or redeployed.
• NAS appliances with many bays: the Seagate IronWolf Pro 28TB (ST28000NT000), a 7200RPM SATA drive from Seagate's NAS line. Check your NAS vendor's compatibility list for this capacity.
• Zone-aware object or archive storage: host-managed SMR such as the WD Ultrastar DC HC680 26TB, or the Western Digital 27TB DC HC680 (WSH722870ALN6L4). The 27TB listing does not state its recording technology, but it belongs to the same HC680 family, so confirm before ordering.
• 30TB and 32TB: the WD Ultrastar DC HC690 30TB and 32TB come in SATA and SAS versions, with several part numbers per capacity (for example WSH723220ALN601 for 32TB SATA and WSH723220AL4201 for 32TB SAS). The listings do not state recording technology. Do not assume CMR: the HC680 in the same Ultrastar series is listed as host-managed SMR, and vendors ship both CMR and SMR drives at the top of their ranges. Tell us the target system and Nexus Compute will confirm recording technology and compatibility for the exact part number before quoting.

What about 26TB drives?

The same split applies at 26TB. The Seagate Exos 26TB (ST26000NM000C) is a 7200RPM SATA enterprise drive. The Western Digital Ultrastar DC HC590 26TB is a 7200RPM SAS drive, the choice for dual-path SAS enclosures or an existing SAS backplane. Western Digital's DC HC5xx drives are its conventional Ultrastar line (the HC560, HC570 and HC580 in our catalog are all listed as CMR), while the DC HC680 is the host-managed SMR model. We also list a Western Digital 26TB SATA drive (0F65672) with limited published detail; ask us to confirm the exact model before you order it for a RAID set.

What changes when drives get this big?

• Rebuild time. A rebuild has to write the replacement drive's full capacity, so a 28TB drive takes far longer to rebuild than a 12TB drive, and the array is exposed the whole time. Use dual parity (RAID 6) or erasure coding rather than RAID 5; our guide to RAID configurations for large datasets walks through the options.
• Performance per terabyte. One set of heads serves more data, so random IOPS per terabyte falls as capacity rises. Drives this size suit capacity tiers, backup targets and archives, not busy transactional data.
• Sector format. Large drives are typically 512e or 4Kn; the HC680 26TB, for example, is 512e. Make sure the operating system and controller handle the format you order.
• Controller support. Older RAID controllers and HBAs may need a firmware update before they recognize the largest drives. Check before the drives arrive.
• Density. Fewer, larger drives reduce slot count and power per terabyte, which is often the real argument for a new capacity tier.

How do I check whether a drive is CMR or SMR before ordering?

Look for the recording technology field on the product page. Nexus Compute tags it where the manufacturer states it, and you can filter straight to CMR drives or host-managed SMR drives. Where a listing does not state it, ask rather than assume. We check recording technology and compatibility against your controller, enclosure or NAS model before dispatch, and quote within 48 business hours.