Dear VSP G400 Unified user,
File system fragmentation over time is typical; this change usually taxes HDD storage pool as I/Os evolve from mainly sequential at initial provisioning to a more random pattern.
Assuming performance statistics confirm the system is random I/O bottlenecked, then adding an SSD tier may improve overall performance, specifically with metadata.
Since your question referenced a HDP to HDT pool change (seemingly using tiered file system approach by expanding a stripeset), I thought the below explanation would be helpful prior to moving forward beyond HDP best practice for HNAS.
It is possible to upgrade a thin provisioned HDP pool to a tiered (HDT) pool, but this option may not provide the expected benefits!
For OLTP/database workload, it is typical to see higher I/O density on a subset of the HDP pages that store metadata, indexes or logs. Tiered pool will use access statistics to promote most active pages on higher storage performance tier. This backend storage optimization works well since write I/O operations perform a data in-place overwrite (change-modify) operation.
But with NAS, overwrite I/O operations append the change-modify results on next available block based on the NAS pool design, which in turn, will render HDP page access profile useless to promote heavily used data. Consequently, HDT pool are not recommended with NAS. Instead, best practice is to use a tiered file system approach instead.
That said, for cold archive (i.e. low change rate - mainly read I/Os), HDT pool is likely a valid solution:
There could be several approaches:
While Hitachi best practices associated to new stripeset are intended for optimal performance, they are not applicable when converting an untiered storage pool to a tiered storage pool. (i.e. using span-tier command). So you can experiment with much smaller amount of flash capacity. For optimal result, consider mirrored SSD parity group instead of RAID6; see best practices for tiered file systems for further details.
Looking forward to hearing what option(s) you elected.