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

[Roman Mamedov <rm@romanrm.net>]

On Wed, 17 Feb 2021 11:27:58 +0800
d tbsky <tbskyd@gmail.com> wrote:

> Hi:
>    I was to use ssd to cache my mdadm-raid5 + lvm storage. 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. I prefer to use the ssd as
> mdadm write journal. is there other reason I should use lvm-cache
> instead of  mdadm write-journal?
> 
> ps: my ssd is intel dc grade, so I think enable write-back mode of
> cache is not problem.

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.

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.

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.

[1] https://lwn.net/Articles/665299/

-- 
With respect,
Roman