My small beginners-roundup about setting up a ZFS-Raid-5 with 3 HDDs and 2 SSDs:
Always use an UPS, a high-quality power supply, HDDs for continuous usage with CMR, SSDs with Power-loss Protection.
# zpool create -f -o ashift=12 <pool> raidz1 <HDD1> <HDD2> <HDD3> log mirror <partition1-on-SSD1> <partition1-on-SSD2> special mirror <partition2-on-SSD1> <partition2-on-SSD2> cache <partition3-on-SSD1> <partition3-on-SSD2>
prepare partitions for log/special/cache on SSDs with parted or gdisk with GPT-partition-tabel ; log on first partition, size half of RAM-size; size of special >= 0,3% of HDD-pool-size; rest for cache.
OPTION log OPTION mirror: this is ZIL (ZFS intent log) write "cache" (log), so use ssd; if not mirrored and ssd fails, loss of about last 5 seconds of written data.
OPTION cache; this is L2ARC (ZFS read cache on disk), so use ssd. more than one <device> -> stripped (there is no mirror OPTION, does not make sense).
OPTION special OPTION mirror: metadata-storage, so use ssd; size >= 0,3% of HDD-pool-size; speeds up more when using many small files on the HDDs; you should use mirror or you can loose much data when special-ssd/partition fails.
