> From: zfs-discuss-bounces@xxxxxxxxxxxxxxx [mailto:zfs-discuss-
> bounces@xxxxxxxxxxxxxxx] On Behalf Of Phil Harman
> I'm wondering whether your HBA has a write through or write back cache
> enabled? The latter might make things very fast, but could put data at
> risk if not sufficiently non-volatile.
He already said he has SSD's for dedicated log. This means the best
solution is to disable WriteBack and just use WriteThrough. Not only is it
more reliable than WriteBack, it's faster.
And I know I've said this many times before, but I don't mind repeating: If
you have slog devices, then surprisingly, it actually hurts performance to
enable the WriteBack on the HBA.
Think of it like this:
Speed of a "naked" disk: 1.0
Speed of a disk with WriteBack: 2.2
Speed of a disk with slog and WB: 2.8
Speed of a disk with slog and no WB: 3.0
Of course those are really rough numbers, that vary by architecture and
usage patterns. But you get the idea. The consistent result is that disk
with slog is the fastest, with WB disabled.
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