On 5/5/22 5:19 AM, Gregory Farnum wrote:
On Wed, May 4, 2022 at 1:25 AM Eneko Lacunza <elacunza@xxxxxxxxx> wrote:
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> <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?
Yep! AFAIK most clusters just use the default pool for replication, barring
mixed media types. *shrug*
This would mean, if I already had a crush rule, like the following, I
wouldn't really need to enable stretch mode:
type replicated
min_size 4
max_size 4
step take room0 ssd
step chooseleaf firstn 2 type host
step emit
step take room1 ssd
step chooseleaf firstn -2 type host
step emit
Am I right about that?
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