[PVE-User] Ceph: Some trouble creating OSD with journal on a sotware raid device...

[Marco Gaiarin]

Mandi! Alwin Antreich
  In chel di` si favelave...

> The ratio for SDD journal to OSD disks, can be done by a simple dd test. Take the write speed of the SSD and divide it
> through the write speed of the OSD, then you will get the max number of disks that can be used on one SSD, without
> expecting the SSD to be the bottleneck.

Wow! Good rule of thumb!


> > So i'm using (software) raid1, confident enought that the penalty of
> > the raid cannot impact so much overral.
> As in the text above, I wouldn't recommend adding another layer in between. Makes harder to troubleshoot.

Probably it is because i'm used to software raid, but seems to me not
really more ''complex''.

I've found also:

	https://www.hastexo.com/resources/hints-and-kinks/solid-state-drives-and-ceph-osd-journals/

but seems old, probably some things changed... anyway it is true:
putting SO and the journals on the same SSD is not a optimal solution.
Also, raid-1 ''double'' the writes.


> You can use files as journal disks, but if recall right, then there was a thread on the ceph mailing list discussing
> this and its limitations.

Anyway seems that jornal on files is not supported, at least by
'pveceph':

 root at vedovanera:~# pveceph createosd /dev/sdb --journal_dev /mnt/journal/sdb
 got unusual device path '/mnt/journal/sdb'

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