hi I have done some tests with the new Allocation Class feature. http://napp-it.org/doc/downloads/special-vdev.pdf
On a basic disk pool an additional special vdev (Optane) where I set special_small_blocks to a value like 64k a filesystem with also a smaller ZFS recsize (64k) gives a massive improvement over other filesystems ex with 128k recsize.
Now I have tried other pool settings like a multi Raid-10 setup where adding a special vdev reduces all performance values for sync/async sequential and random read/write values. From a first view ist seems that performance is like the raw performance of the special vdev (Intel P3600 in this case) and there is no improvement from Arc readcaching or ram writecaching as the performance values were better without special vdev in this benchmark.
Can someone confirm this behaviour (that special vdev do not use Arc or ram write caching). While this would help for a lot of workloads with low ram or given iops needs, this is essential to care about when designing a pool layout.
gea @napp-it.org ------------------------------------------ illumos: illumos-discuss Permalink: https://illumos.topicbox.com/groups/discuss/T7dbc47cd249baa9c-M1fcb89e9f96e22416f2f5e62 Delivery options: https://illumos.topicbox.com/groups/discuss/subscription
