Hi,
I meant that as long as your third node is offline, your PGs will stay
degraded until the node comes back because there's no recovery target.
Your cluster will serve I/O though, yes.
Even when another node failed, I still have one node in read-only
mode while recovery node 2 and 3. Sure, this is not optimal, but
it's okay for my use-case.
You won't have MON quorum if you lose a second node. Not sure what you
mean by "one node in read-only mode", you need two intact replicas to
server I/O (min_size 2) and a MON quorum, so no, you can't lose two
nodes.
Zitat von Daniel Vogelbacher <daniel@xxxxxxxxxxxxxx>:
Hi Eugen,
On 3/11/25 16:48, Eugen Block wrote:
Hi Daniel,
the first thing to mention is, while min_size 2/size 3 is good,
having only three nodes leaves the cluster without any options to
recover in case of a node failure. So it 's recommended to use at
least four nodes.
What exactly do you mean with "without any options to recover"? From
my understanding, with min_size=2 I can still operate the cluster
with 2 healthy nodes in read-write I/O during recovery of the third
node.
Even when another node failed, I still have one node in read-only
mode while recovery node 2 and 3. Sure, this is not optimal, but
it's okay for my use-case.
You have to be aware that the hybrid rule only gives you
performance advantages for read requests (from primary the OSD).
Writing is only completed when all replicas have acked the write,
so your clients will be waiting for the HDDs to ack.
The 5 TB are not wasted if you have other pools utilizing HDDs.
Regards,
Eugen
Zitat von Daniel Vogelbacher <daniel@xxxxxxxxxxxxxx>:
Hi,
I want to setup a 3-node Ceph cluster with fault domain configured
to "host".
Each node should be equipped with:
6x SAS3 HDD 12TB
1x SAS3 SSD 7TB (should be extended to 2x7 later)
The ceph configuration should be size=3, min_size=2. All nodes are
connected with 2x10Gbit (LACP).
I want to use different CRUSH rules for different pools. CephFS
and low priority/IO VMs stored on RBD should use only HDD drives
with default replication CRUSH rule.
For high priority VMs, I want to create another RBD data pool
which uses a modified CRUSH replication rule:
|# Hybrid storage policy rule hybrid { ruleset 2 type replicated
step take ssd step chooseleaf firstn 1 type host step emit step
take hdd step chooseleaf firstn -1 type host step emit } |
|For pools using this hybrid rule, PGs are stored on one SSD
(primary) and two HDD (secondary) devices. But these have
different sizes in my hardware setup. What happens with the
remaining disk space (12-7=5) 5GB on the secondary devices? Is it
just unusable, or will ceph use it for other pools with default
replication? In any case, I don't bother about these 5GB, just
want to know how it works. For the above setup, can you recommend
any important configuration settings and should I modify the OSD
weighting? Thanks. |-- Best regards / Mit freundlichen Grüßen
Daniel Vogelbacher
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--
Best regards / Mit freundlichen Grüßen
Daniel Vogelbacher
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