Hi Yoann,
So you need to think about failure domains.
If you put all the DB's on one SSD and all the WAL's on another SSD then a failure of either of those SSD's will result in a failure of all the OSD's behind them. So in this case all 10 OSD's would have failed.
Splitting it to 5 OSD's you have RocksDb and WAL on each SSD this then results in a failure of an SSD only impacting 5 OSD's.
A failure of an SSD will take down all the OSD's that are behind that SSD.
That's one of the reasons I would always say you need 1 nodes worth of spare capacity in the cluster to allow for automated re-builds to happen.
As for your EC 7+5 I would have gone for some thing like 8+3 as then you have a spare node active in the cluster and can still provide full protection in the event of a failure of a node.
Think about software updates that require a reboot of a node. Any data written during that time will need recovering to bring it back to full protection where as if you have a spare node then that data could be written and not require a later recovery.
Darren
On 03/09/2019, 10:29, "Yoann Moulin" <yoann.moulin@xxxxxxx> wrote:
Hello,
I am deploying a new Nautilus cluster and I would like to know what would be the best OSD's scenario config in this case :
10x 6TB Disk OSDs (data)
2x 480G SSD previously used for journal and can be used for WAL and/or DB
Is it better to put all WAL on one SSD and all DBs on the other one? Or put WAL and DB of the first 5 OSDs on the first SSD and the 5 others on
the second one.
A more general question, what is the impact on an OSD if we lose the WAL? The DB? Both?
I plan to use EC 7+5 on 12 servers and I am OK if I lose one server temporarily. I have spare servers and I can easily add another one in this
cluster.
To deploy this cluster, I use ceph-ansible (stable-4.0). I am not sure how to configure the playbook to use SSD and disks with LVM.
https://github.com/ceph/ceph-ansible/blob/master/docs/source/osds/scenarios.rst
Is this good?
osd_objectstore: bluestore
lvm_volumes:
- data: data-lv1
data_vg: data-vg1
db: db-lv1
db_vg: db-vg1
wal: wal-lv1
wal_vg: wal-vg1
- data: data-lv2
data_vg: data-vg2
db: db-lv2
db_vg: db-vg2
wal: wal-lv2
wal_vg: wal-vg2
Is it possible to let the playbook configure LVM for each disk in a mixed case? It looks like I must configure LVM before running the playbook
but I am not sure if I missed something.
Is wal_vg and db_vg can be identical (on VG per SSD shared with multiple OSDs)?
Thanks for your help.
Best regards,
--
Yoann Moulin
EPFL IC-IT
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