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

I was to use ssd to cache my mdadm-raid5 + lvm storage.

Not that sure that layering MDADM on top of DM/LVM2 is always a
good idea, I tend to prefer to keep things simple.

but I wonder if I should use them as lvm-cache or mdadm write
journal.  lvm-cache has benefits that it can do also
read-cache. but I wonder if full-stripe write is the key point
I need.

It depends on your load; does the small chance of RAID5 "write
hole" matter to your load? MDRAID has been used for a long time
without having a write journal, as the "write hole" issue
happens rarely and does not always matter. Anyhow with a write
journal, slow "resyncs" are avoided, which may also be
convenient (using the journal as a write buffer is not not that
coherent).

Also, the MDRAID write journal is usually (and should be) a lot
smaller than a whole flash SSD unit, so you can use a small part
of a flash SSD for write journaling and most of it for caching.

Whether caching is useful, and whether DM/LVM2 caching in
particular is useful, depends a lot on the specific load.

Apart from DM/LVM2 caching there is also "bcache", and there is
a new and fairly reliable filesystem type, "bcachefs", that
integrates it and gives quite neatly multiple tiers of storage
(it also has some RAID aspects, but those can be ignored).

https://www.reddit.com/r/bcachefs/comments/l44lmj/list_of_some_useful_links_for_bcachefs/