When specifying servers, like (I would assume) many engineers who aren't experts in storage, I'll generally play it safe (and perhaps be a slave to marketing) by standardising on a minimum of 10k SAS drives (and therefore are "enterprise"-grade with a 24x7 duty cycle, etc) for "system" data (usually OS and sometimes apps), and reserve the use of 7.2k mid/nearline drives for storage of non-system data where performance isn't a significant factor. This is all assuming 2.5" (SFF) disks, as 3.5" (LFF) disks are only really relevant for high-capacity, low IOPs requirements.
In situations where there isn't a massive amount of non-system data, I'll generally place it on the same disks/array as the system data, meaning the server only has 10k SAS drives (generally a "One Big RAID10" type of setup these days). Only if the size of the non-system data is significant do I usually consider putting it on a separate array of 7.2k mid/nearline disks to keep the cost/GB down.
This has lead me to wonder: in some situations, could those 10k disks in the RAID10 array have been replaced with 7.2k disks without any significant negative consequences? In other words, am I sometimes over-spec'ing (and keeping the hardware vendors happy) by sticking to a minimum of 10k "enterprise" grade disks, or is there a good reason to always stick to that as a minimum?
For example, take a server that acts as a hypervisor with a couple of VMs for a typical small company (say 50 users). The company has average I/O patterns with no special requirements. Typical 9-5, Mon-Fri office, with backups running for a couple of hours a night. The VMs could perhaps be a DC and a file/print/app server. The server has a RAID10 array with 6 disks to store all the data (system and non-system data). To my non-expert eye, it looks as though mid/nearline disks may do just fine. Taking HP disks as an example:
- Workload: Midline disks are rated for <40% workload. With the office only open for 9 hours a day and average I/O during that period unlikely to be anywhere near maximum, it seems unlikely workload would go over 40%. Even with a couple of hours of intense I/O at night for backups, my guess is it would still be below 40%
- Speed: Although the disks are only 7.2k, performance is improved by spreading it across six disks
So, my question: is it sensible to stick a minimum of 10k SAS drives, or are 7.2k midline/nearline disks actually more than adequate in many situations? If so, how do I gauge where the line is and avoid being a slave to ignorance by playing it safe?
My experience is mostly with HP servers, so the above may have a bit an HP slant to it, but I would assume the principles are fairly vendor independent.