FREQUENTLY ASKED QUESTIONS
Q: How does Exos® 2X provide up to 2× the performance of a standard singleactuator hard drive?
A: Exos 2X can demonstrate up to 2× the performance of a standard single actuator hard drive because it has two independent actuators and data paths, allowing for concurrent I/O
streams to and from the host.
Q: How does an Exos® 2X SATA configuration differ from a SAS configuration?
A: For the SAS configuration, each actuator is assigned to a logical unit number (LUN 0
and LUN 1). For example, one 18TB SAS drive will present itself to the operating system as two 9TB devices that the operating system can address independently, as it would with any other HDD.
The Exos® 2X SATA configuration will present itself to the operating system as one logical device since SATA does not support the concept of LUNs. The user must be aware that the first 50% of the logical block addresses (LBAs) on the device correspond to one actuator and the second 50% of the LBAs correspond to the other actuator. With both configurations, the user must send commands to both actuators concurrently to see the expected performance benefits.
Q: How do you identify which LBAs correspond to each actuator on an Exos® 2X
SATA device?
A: Seagate® has worked with the T13 ATA committee to propose and implement a new log page for SATA the Concurrent Positioning Ranges log page 47h identifies the number of LBA ranges (in this case, actuators) within a device. For each LBA range the log page specifies the lowest LBA and the number of LBAs. As a reminder, since LBA numbering starts at zero, the last LBA of either range will be the lowest LBA + the number of LBAs 1.
Q: How do you identify these LBA ranges in Linux?
A: In Linux Kernel 5.19, the independent ranges can be found in /sys/block//
queue/independent_access_range. There is one sub-directory per actuator, starting with the primary at 0. The nr_sectors field reports how many sectors are managed by this actuator, and sector is the offset for the first sector. Sectors then run contiguously to the start of the next actuator's range.
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Q: What workloads show the best performance benefits over a single actuator?
A: Exos 2X was designed for hyperscale workloads that focus on low queue-depth random read operations (low queue-depths keep command latencies low) and large transfer size sequential operations. The highest performance gains over a single actuator will be found during high transfer size sequential reads/writes (128KB transfers and larger), random reads
(all transfer sizes), and random writes (128KB transfers and larger).
Q: What performance benchmarks have shown a good benefit over a single actuator?
A: Microsoft Jetstress, sequential throughput testing using fio (simulating backup operations or streaming), random read IOPS testing using fio (simulating hyperscale and CDN
workloads), and more benchmarks are under investigation.
Q: How do you create a script in fio to test a dual-actuator SATA?
A: Within a fio job file, you can specify two threads one for the first actuator (usingÿü
size=50%) and one for the second actuator (using offset=50%). An example of a simple random read script to run 4KB transfers with a queue depth of 1 is provided here for Linux and Windows:
Linux fio: Random Read 4KB Q1
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