What is the rule of thumb in performance characteristics and differences between 7200rpm (SATA/SAS) and 15000rpm (SAS) hard disc drives of the same generation?
It's a case of math. I/O-per-second is determined by the following calculation:
Random I/O = 1000/ (average latency + seek time)
For a 7.2K RPM drive, a seek-time of 8.5ms and latency of 4.16 gives an IOPS number of 78.
For a 15K RPM drive, a seek-time of 2.6ms and latency of 2.0ms gives an IOPS number of 217 .
These are just examples based on a selection of current (as of this writing) drives from Seagate.
Interface (SAS vs SATA) doesn't impact these numbers meaningfully. The RPMs are what determine the performance. Sequential I/O performance is a different number, but most server I/O these days is significantly random.
As you can see, the performance difference between the two tiers of speed can be quite significant.
In short, they have different characteristics and purposes...
7200rpm disks are bigger and slower. 10000rpm and 15000rpm disks are smaller and faster. The latter are meant for high transactional and high random I/O workloads. The former are better for sequential transfer and high capacity requirements.
From HP's documentation:
HP Entry (SATA 7.2k) drives have the lowest unit cost and give you a basic level of reliability and performance. They are best suited for non-mission-critical environments where I/O workloads are 40 percent or less. They are typically best suited for internal/archival storage or as boot drives for entry-level servers.
HP Midline (SAS 7.2k) drives give you larger capacity and greater reliability than Entry drives. Midline drives are more resistant to rotational and operational vibration, so they are better suited for use in multi-drive configurations. HP Midline drives are for high-capacity applications such as external storage that may require increased reliability. Like Entry drives, however, Midline drives are for use in moderate workload environments. We do not recommend Entry or Midline drives for mission-critical applications.
HP Enterprise (SAS 10k/15k) drives give you maximum reliability, highest performance, scalability, and error management under the most demanding conditions. They are the only HP drives designed for use at unconstrained I/O workloads. They are for mission-critical applications such as large databases, e-mail servers, and back-office.
From a value point of view, one can often get better real world performance out of more SATA 7.2k drives than fewer SAS 15k drives with the price being similar. You'll notice that larger buildouts like Backblaze use SATA and that's because the value is so much better and if you use multiple SATA drives, the performance can match for typical storage loads.