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I'm setting up ZFS (through FreeNAS) with RAIDZ1 on a server with 4 x WD Red SATA HDDs (connected through a PERC H330 in HBA mode).

The server is hooked to a UPS.

For ZFS and in this setup, does it make sense to enable HD cache of each disk, or is this very dangerous despite the UPS?

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You should definitely enable the disk cache.

The rationale is that ZFS assumes enabled disk cache and so flushes any critical writes (ie: sync write and uberblock rewrite) via appropriate and specific SATA/SAS commands (ATA FLUSH, FUAs, etc).

Leaving the disk cache enabled permits to capitalize on the write-combining capability of modern disks without impact on pool reliability.

This obviously assumes that your disks actually honor the cache flush command, which is the norm for modern (post-2006) disks. In the rare cache your disks lie about cache flushing, then you should disable it.

As additional information, I suggest you reading the zfs_nocacheflush tunable description:

ZFS uses barriers (volatile cache flush commands) to ensure data is committed to permanent media by devices. This ensures consistent on-media state for devices where caches are volatile (eg HDDs).

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You can if you want. It won't make a big difference. ZFS leverages a portion of RAM for write cache and flushes to disk periodically.

With 4 disks, this sounds like a small installation, so benchmark both and see if there's even a noticeable benefit first.

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