Here's a quick one that I was reminded of today...
The EasyTier 3 feature was introduced in 188.8.131.52 and introduced a number of new EasyTier features, but a few of them relied on knowledge of the maximum throughput and IOPs capability for each managed disk.
On Storwize RAID arrays, this was a fairly easy problem to solve. We could measure the performance of individual drives and then calculate the managed disk's capabilities because we know how many drives there are and what type of RAID we are using.
However for SAN attached managed disks we have no way of knowing the maximum capabilities for a managed disk. In fact it's often the case that two different managed disks may share some resources meaning that we can't even reliably measure the maximums. So for SAN attached managed disks we created a small number of "generic" profiles. These generic profiles can be selected using chmdisk -easytierload <default|low|medium|high|very_high>
It's actually quite hard to explain to you exactly when you should use each of these different tiers - so I'll try and do that in a future blog post, however there is one really really simple piece of advice I can give:
- If you are using FlashSystem behind SVC in an easytier pool - you should set the easytierload setting of all of the FlashSystem managed disks to very_high
If this setting is set too low then the EasyTier software may decide that the managed disk has reached it's maximum throughput. When the managed disk has reached it's maximum throughput then this is treated roughly the same as if the managed disk was 100% full - we don't put anything more on the managed disk without first taking something else off. This means that you may not be using all of the managed disk's performance and capacity.
UPDATE: We will automatically configure "very_high" for FlashSystems managed disks which are newly created and presented to an SVC running V184.108.40.206 or later. If you configured a managed disk on 220.127.116.11 and then upgrade to 18.104.22.168, the load setting will not change automatically.
If you are still here and you'd really like to know about using this load parameter on other SSD or Flash managed disks - not just FlashSystem, then it does require that you know about using STAT to look at the EasyTier statistics.
If the STAT output shows evidence of warm demote from your SSD tier SAN attached managed disk when the managed disk is not 100% full and the response time of the managed disk doesn't indicate any overloading, then you *might* want to increase the EasyTier load setting so that we will use more of the capacity. However overloading does cause performance problems, so there are risks associated with this.