Nstripes optimal RAIN N+2 settings for number of FSTs

Hi all,

What is the ideal setting for RAIN N+2 and nstripes.
Our setup consists of 9 FSTs + 3 disk enclosures shared between 3 FSTs each.

A single FST sees 27 disks, therefore we have 27 groups configured.
We want to go with a raid6 RAIN, i.e n+2 redundancy. Since we have 9 FSTs, we though of going a nstripes setting of (7+2 =) 9.

During our tests we were taking down one of the FST nodes, to simulate failure. This resulted in reads being OK (expected), but writes failing (initially a surprise, but actually expected, as not enough filesystems are available within each group).

How can we improve upon this (or is it considered ok to be read only when 1 FST is down - I don’t really like that thought). Our current approach now is to go with nstripes=7 (i.e. 5+2), which will cause more overhead, but will allow for full availability with 2 nodes down or 2 disks failing in the same group.

What are you opinions or current practices?