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

[pg@mdraid.list.sabi.co.UK (Peter Grandi)]

> I thought journal write-back mode should use large ssd
> space,like bcache which will prevent random write at all cost.

The write journal is supposed to buffer a few stripes to avoid
the write hole. Consider the case of a 2-drive write journal
arrangement: you would be effectively adding a RAID1 component
to your RAID5 set for recently updated data. Then why use RAID5?
Also consider the size of journals for filesystem types that
have it: typically it is 32MiB-128MiB.

> but reading the document again, it said "The flush conditions
> could be free in-kernel memory cache space is low".

That's another issue with the Linux default for the buffer
system, it usually buffers too much if there is no 'sync'.

> since the memory won't be too large compare to normal ssd
> disk,

I am not sure I understand why that is relevant, what happens
there depends on 'sync' behaviour and the filesystem and buffer
cache flushing interval if any.

> maybe a small optane ssd is best for mdadm write-journal.

The reasoning before this I don't quite understand, but Optane
is a very good choice for a persistent write buffer, as it is
not volatile and has much faster and smaller writes than flash
chips.