You may hear Zadara Support or your Solutions Architect recommend that you expand your classic VPSA storage Pool by a minimum of 4 drives ( 2 raid groups), this article should give insight as to why that advice is given.
Note: This applies to the Zadara classic VPSA not to our Flash Array VPSA.
___________________________________________________________________________________________
VPSA storage is built on a number of components, in this article we'll concentrate on the storage elements.
Each VPSA will have at least one storage Pool, with each Pool being a collection of physical drives arranged into raid groups and aggregated to form a Pool. A raid group being 2 drives of the same size and technology ( HDD+HDD , SSD+SSD ... etc). The concept of RAID will not be discussed in the scope of this article and can be researched online.
VPSA Pool
A Pool is a collection of raid groups, we use RAID 10 - stripe+mirror. Each single raid group has 2 drives. Whilst you can create a Pool from a single raid group, it will only perform as fast as that single raid group and the physical drive components that form that raid group.
Creating (or expanding) a Pool with multiple raid groups has a magnifying effect; the raid groups are aggregated and work concurrently.
Raid Group Stripe
As an example; we have 3 raid groups and if we write a stripe of data across these 3 groups we will see 3 times the bandwidth compared to a single raid group.
If a single raid group has a 200 IOPs capability then with a stripe across 3 raid groups we would see around a 600 IOP capability with data being written to the raid groups and drives operating concurrently. This gives us better performance. In the context of the VPSA the 3 raid groups would need to be added to a Pool at the same time to act in a collaborative stripe operation.
Raid Group Concatenation
Again we have a raid group comprised of 2 drives.
In a VPSA the Pool can be composed of a single raid group (not recommended) or at time of expansion just one raid group can be added to an existing Pool (again not recommended). Usually this situation arises when the Pool capacity is low and in need of expansion. The single raid group (drive pair) runs hot as there are no other raid groups to operate concurrently. A single raid group may achieve 200 IOPs at best will not see any raid benefit. Also, if expanding an existing striped pool by a single raid group the single raid group is added as concatenated and whilst it provides extra capacity the performance will not match the previously striped configuration.
If the Pool started life as a single raid group and grew slowly then the Pool might be comprised of multiple concatenated raid groups and the usage term waterfall can be used to describe it's operation. ( Raid group 1 fills up, a second raid group is added and this fills up and then a third raid group is added and this fills up). The user never sees a raid benefit as each raid group operates without concurrency.
How to Tell
In a VPSA you can tell if your Pool is striped or concatenated by checking the Raid Groups tab from the Pools screen and picking the Segments View button on the bottom ribbon bar.
The first screen shows a VPSA that started with single raid group in the Pool so no stripe is available.
The Pool was then expanded by one more raid group and this continues in concatenated mode.
Unfortunately, once a Pool has entered concatenated mode it is not possible to return the Pool to a striped configuration and raid groups can only be added in concatenated mode. However, other options are available to address the situation. (e.g. online volume migration to a new pool )
___________________________________________________________________________________________
When creating a Pool manually ensure the Striped tick-box is checked.
The resulting Pool will be striped across the raid groups providing optimal performance.
To maintain the stripe a minimum of 2 raid groups would need to be added. Alternatively for a full Pool stripe we can create a completely new Pool with full stripe and then online migrate volumes to it.
Conclusion
For optimal performance go striped.