Re: use ssd as write-journal or lvm-cache?

Roman Mamedov [off-list ref]

On Wed, 17 Feb 2021 11:27:58 +0800
Why not both? It's not like you have to use the entire SSD for one or the
other. And it's very unlikely anything will be bottlenecked by concurrent
access to the SSD from both mechanisms.

   if I use both, then data may write twice to the same ssd, it seems waste.

Choosing one, I would prefer LVM caching, since it also gives benefit for
reads. And the mdadm write journal feature sounds[1] more like of a
reliability, not a performance enhancement.

   at final stage data write to disk with any kind of cache. so if the
data can write to disk with optimized method,
it seems speed up the whole thing. read-cache is fine, but I don't
know how much benefit it will bring.

In any case, in order to not add a single point of failure to the array,
better rely not on SSD being a "datacenter" one (anything can fail), but use a
RAID1 of two SSDs.

   yes raid1 is must. "datacenter" ssd can protect the ssd-cache so it
won't suffer with power outage. so I think I can enable write-back
mode safely.