Power-loss protection

Many storage servers tell the Mac a write is done as soon as it’s in their memory, and save it to their drives moments later. If the server loses power or restarts in between, those writes are lost, and the disk can be damaged. Power-loss protection keeps your most recent writes safe through that, as long as your Mac stays on.

It’s a setting for each disk, on the disk’s page, in the Connection card.

The choices

Choice	What it does
Automatic (the default)	On, unless the server says the writes it accepts survive a power loss.
On	Always on.
Off	Faster writes, but if the server crashes or loses power, recent writes can be lost and the disk damaged.

Under the menu, a note says what Automatic chose and why:

• On: the server can lose writes it has accepted if it loses power. TrueNAS says this about its disks, so Automatic is on for TrueNAS.
• On: the server doesn’t say whether its writes survive a power loss.
• Off: the server says the writes it accepts survive a power loss.

Change it

1. Open the disk’s page in BeamDisk’s window.
2. In the Connection card, choose Automatic, On or Off next to Power-loss protection.
3. The change applies the next time the disk connects: at your next login, or after Disconnect All and Connect All. Until then the note says The change applies the next time this disk connects.

Should you turn it off?

Only for a disk whose contents you can afford to lose. If your server’s writes do survive a power loss and it reports that, Automatic already turns protection off.

What it gains, measured on TrueNAS over 10 GbE: turning it off made long sequential writes about 11% faster on an SSD pool (about 920 MB/s against 830 MB/s) and about 9% faster on a hard-drive pool, with an occasional pause of several seconds. What it costs is the protection: if the server crashes or loses power, recent writes can be lost and the disk damaged.

So the default stays on for TrueNAS, on SSDs and hard drives alike.

Related

• When the server stops responding
• Set up TrueNAS