Yes, when creating an EC profile, it automatically creates a CRUSH rule
specific for that EC profile. You are also correct that 2+1 doesn't really
have any resiliency built in. 2+2 would allow 1 node to go down while
still having your data accessible. It will use 2x data to raw as opposed
to the 1.5x of 2+1, but it gives you resiliency. The example in your
command of 3+2 is not possible with your setup. May I ask why you want EC
on such a small OSD count? I'm guessing to not use as much storage on your
SSDs, but I would just suggest going with replica with such a small
cluster. If you have a larger node/OSD count, then you can start seeing if
EC is right for your use case, but if this is production data... I wouldn't
risk it.
When setting the crush rule, it wants the name of it, ssdrule, not 2.
On Thu, Nov 1, 2018 at 1:34 PM Steven Vacaroaia <ste...@gmail.com> wrote:
> Hi,
>
> I am trying to create an EC pool on my SSD based OSDs
> and will appreciate if someone clarify / provide advice about the following
>
> - best K + M combination for 4 hosts one OSD per host
> My understanding is that K+M< OSD but using K=2, M=1 does not provide
> any redundancy
> ( as soon as 1 OSD is down, you cannot write to the pool)
> Am I right ?
>
> - assigning crush_rule as per documentation does not seem to work
> If I provide all the crush rule details when I create the EC profile, the
> PGs are being placed on SSD OSDs AND a crush rule is automatically create
> Is that the right/new way of doing it ?
> EXAMPLE
> ceph osd erasure-code-profile set erasureISA crush-failure-domain=osd k=3
> m=2 crush-root=ssds plugin=isa technique=cauchy crush-device-class=ssd
>
>
> [root@osd01 ~]# ceph osd crush rule ls
> replicated_rule
> erasure-code
> ssdrule
> [root@osd01 ~]# ceph osd crush rule dump ssdrule
> {
> "rule_id": 2,
> "rule_name": "ssdrule",
> "ruleset": 2,
> "type": 1,
> "min_size": 1,
> "max_size": 10,
> "steps": [
> {
> "op": "take",
> "item": -4,
> "item_name": "ssds"
> },
> {
> "op": "chooseleaf_firstn",
> "num": 0,
> "type": "host"
> },
> {
> "op": "emit"
> }
> ]
> }
>
> [root@osd01 ~]# ceph osd pool set test crush_rule 2
> Error ENOENT: crush rule 2 does not exist
>
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