It was almost a year ago when Seagate unveiled the specifications for the Mach.2 drive and planned to launch only SAS drives that would present themselves to the host as 2 LUNs. Since SATA 's specifications do not allow for multiple LUNs, there was no plan to bring a SATA drive to market.
A year later they have now launched dual actuator drives with both SATA and SAS interfaces. With SAS the drive still presents itself as 2 LUNs, but with SATA the internals are managed by the drive and the SATA drive presents itself to the host as a single 16/18TB drive.
Before we look at the drive specifications, a little background on the benefits of Dual Actuator (or in some years Multi Actuators) drives.
As HDD capacities increase and transfer rates stagnate, dual actuator drives solve this problem.
HDD recording technologies such as energy- assisted magnetic recording have given us larger HDDs - at the time of writing, 24 TB and very soon there will be 28-30 TB HDDs.However, the recording breakthroughs have had no impact on transfer rates. The end result: we have larger hard drives that are just as slow as their smaller counterparts. To avoid this bottleneck, the transfer rate must be increased. By using 2 independent actuators and parallel read/write, transfer rates could theoretically be doubled - just like a RAID 0 setup. This is what the dual actuator drive tries to achieve.
SAS is represented as 2 LUNS, SATA as a single drive
Seagate's literature talks about 2 LUNs for the SAS drive, so your host will see 2 logical units, but SATA cannot have multiple LUNS, so SATA works similarly to RAID0, where a single block device is represented to OS.Part numbers and encryption options
For 2X18, There are 2 capacities listed on the datasheet - 16TB and 18TB. SATA has only 1 single variant, Standard FastFormat. It is shipped as 512e but can be formatted to 4Kn. SAS will have 2 variants - the standard model and the SED model , both with FastFormat. Currently there are no SED-FIPS models.
| Part Number | Capacity | Platform | Interface | Encryption | Format | ||||||
| ST18000NM0272 | 18 TB | 2X18 | SAS | None | FastFormat | ||||||
| ST18000NM0012 | 18TB | 2X18 | SAS | SED | FastFormat | ||||||
| ST18000NM0092 | 18TB | 2X18 | SATA | None | FastFormat | ||||||
| ST16000NM0002 | 16TB | 2X18 | SAS | none | FastFormat | ||||||
| ST16000NM0012 | 16TB | 2X18 | SAS | SED | FastFormat | ||||||
| ST16000NM0092 | 16TB | 2X18 | SATA | None | FastFormat |
Almost Double the transfer rates, double the random read IOPS
let’s compare X18 ( single Actuator ) drive performance with 2X18 ( Dual Actuator drive).
| X18 Single Actuator | 2X18 Dual Actuator | |
| Max Suistained Transfer rate OD MB/s | 270 | 545-554 |
| Random Read 4K QD16 IOPS | 170 | 304 |
| Random Write 4K QD16 IOPS | 550 | 560 |
| Average Latency (ms) | 4.16 | 4.16 |
- Max Suistained xfr rate has almost doubled. SATA has a marginally lower XFR rate then SAS.
- Random Read IOPS have also almost doubled
- Random write IOPS are almost the same. Most probable reason is that both the X18 and 2X18 have 256MB cache. Write requests are writtent to the cache first and reported to be written.
- Average latency is also the same for both. Both have the same 7200RPM so the rotational latency is the same for entire Exos series.
Going forward as the hard drive capacities increase,vendors will have to find ways to increase the transfer rates. Its just a matter of time before other vendors come up with multi actuator drives also as nobody wants bigger, slower drives.
