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Subject: Mirrored SSDs for ZIL/SLOG - safety, flushing, capacitors
From: javocado <javocado@gmail.com>
To: FreeBSD Filesystems <freebsd-fs@freebsd.org>
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I'm setting up a SLOG to improve performance on my ZFS-based fileserver. I
plan to use 2 mirrored SSD's to create the log device. Before I continue,
I have some specific questions how to approach which hardware to use.

Primarily, I'm concerned with what damage/corruption may occur when there's
a power loss.

1. if some data is not successfully written to the SSD-SLOG device as a
result of a power loss, is this a huge deal? Or is it just a loss of some
non-fatal meta data?

to help avoid any loss:

2. should I only use SSDs with capacitors? does anyone recommend any? I'm
seeing good things about the Intel s3500

3. if only one of the SSDs has a capacitor (I plan to use 2 different SSDs
to avoid simultaneous/similar failure rates) what might that do to the SLOG
mirror after a power loss? Will it know which SSD had the capacitor and
thus slightly-newer data to remirror from?

4. flushing: does anyone know if (and which) SSDs are respecting and
flushing data to media when told?

Thanks!