Hi Gregory,
El 3/5/22 a las 22:30, Gregory Farnum escribió:
On Mon, Apr 25, 2022 at 12:57 AM Eneko Lacunza<elacunza@xxxxxxxxx> wrote:
We're looking to deploy a stretch cluster for a 2-CPD deployment. I have
read the following docs:
https://docs.ceph.com/en/latest/rados/operations/stretch-mode/#stretch-clusters
I have some questions:
- Can we have multiple pools in a stretch cluster?
Yes.
- Can we have multiple different crush rules in a stretch cluster? I'm
asking this because the command for stretch mode activation asks for a
rule...
Right, so what happens there is any pool with a default rule gets
switched to the specified CRUSH rule. That doesn't stop you from
changing the rule after engaging stretch mode, or giving it a
non-default rule ahead of time. You just have to be careful to make
sure it satisfies the stretch mode rules about placing across data
centers.
So the only purpose of the "stretch_rule" param to "enable_stretch_mode"
is replacing default replicated rule?
If so, when stretch mode is activated, the described special behaviour
applies to all crush rules/pools:
- the OSDs will only take PGs active when they peer across data centers
(or whatever other CRUSH bucket type you specified), assuming both are alive
- Pools will increase in size from the default 3 to 4, expecting 2
copies in each site (for size=2 pools will it increase to 4 too?)
Is this accurate?
We want to have different purpose pools on this Ceph cluster:
- Important VM disks, with 2 copies in each DC (SSD class)
- Ephemeral VM disks, with just 2 copies overall (SSD class)
- Backup data in just one DC (HDD class).
Objective of the 2-DC deployment is disaster recovery, HA isn't
required, but I'll take it if deployment is reasonable :-) .
I'm leery of this for the reasons described in the docs — if you don't
have 2 replicas per site, you lose data availability every time an OSD
goes down for any reason (or else you have a window while recovery
happens where the data is not physically available in both sites,
which rather negates the purpose).
Is this because the following: "the OSDs will only take PGs active when
they peer across data centers (or whatever other CRUSH bucket type you
specified), assuming both are alive"?
This conclusion wasn't obvious to me, but after your reply and a third
read, now seems expected :-)
Thanks for your comments. I'll be doing some tests, will write back if I
find something unexpected (to me?) :-)
Cheers
Eneko Lacunza
Zuzendari teknikoa | Director técnico
Binovo IT Human Project
Tel. +34 943 569 206 |https://www.binovo.es
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