> 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/
