I was wondering if it makes sense to have FLASH and NL-SAS two tier pool. What are the advantages and disadvantages of it? The best practices says to AVOID it if the skew is unknown, but does not say you cannot have it at all. I cannot really understand what is the skew of the array, and how to find out what it is in the existing environment?
Appreciate your input.
You might want to take advantage of the Mitrend site - this site is used by customers to submit a set of spcollects and a NAR file or sequentail NAR files (in a single zip file) and get a report back detailing the performance of the array. I think it may include a measure of skew.
You can access the site at:
The concern I have with that particular Pool layout is the big difference in performance between the 2 tiers.
Measuring "skew" is a way to measure how much of your data generates the total amount of iops. IE is 10% of the data footprint responsible for generating 80% of the iops load ?
If your extreme performance tier (ssd) is not sized appropriately you may find the the moment the skew exceeds it, and pulls data out of the capacity tier for an active workload, is the moment when the performance can fall right off "the cliff".
In a 3 tier pool, the SAS (performance) tier makes that fall less drastic and gives a better buffer for the data to land on.
Consider using some of the higher capacity 10k SAS drives, the 1.2TB are a good compromise. Obviously this depends on your budget and requirements though.
A Skew is the small percentage of overall capacity of the array, which is responsible for most of the I/O activity on the box. You can find a Skew within a Pool, a small percentage of a LUN and so on... Basically it is those spindles on the drives which are being utilized more most of the time comparatively others.
On the other hand talking about NL-SAS and FLASH the advantage could be faster access to data if it on Cache/Flash drives and disadvantage is read miss in that case data has to be fetched from NL-SAS a bit slower but if you have 3 tiers they are equally balanced.