On 08/15/2014 06:24 AM, Wido den Hollander wrote:
On 08/15/2014 12:23 PM, Loic Dachary wrote:
Hi Erik,
On 15/08/2014 11:54, Erik Logtenberg wrote:
Hi,
With EC pools in Ceph you are free to choose any K and M parameters you
like. The documentation explains what K and M do, so far so good.
Now, there are certain combinations of K and M that appear to have more
or less the same result. Do any of these combinations have pro's and
con's that I should consider and/or are there best practices for
choosing the right K/M-parameters?
Loic might have a better anwser, but I think that the more segments (K)
you have, the heavier recovery. You have to contact more OSDs to
reconstruct the whole object so that involves more disks doing seeks.
I heard sombody from Fujitsu say that he thought 8/3 was best for most
situations. That wasn't with Ceph though, but with a different system
which implemented Erasure Coding.
Performance is definitely lower with more segments in Ceph. I kind of
gravitate toward 4/2 or 6/2, though that's just my own preference.
For instance, if I choose K = 3 and M = 2, then pg's in this pool will
use 5 OSD's and sustain the loss of 2 OSD's. There is 40% overhead in
this configuration.
Now, if I were to choose K = 6 and M = 4, I would end up with pg's that
use 10 OSD's and sustain the loss of 4 OSD's, which is statistically not
so much different from the first configuration. Also there is the same
40% overhead.
Although I don't have numbers in mind, I think the odds of loosing two
OSD simultaneously are a lot smaller than the odds of loosing four OSD
simultaneously. Or am I misunderstanding you when you write
"statistically not so much different from the first configuration" ?
Loosing two smaller then loosing four? Is that correct or did you mean
it the other way around?
I'd say that loosing four OSDs simultaneously is less likely to happen
then two simultaneously.
This is true, though the more disks you spread your objects across, the
higher likelihood that any given object will be affected by a lost OSD.
The extreme case being that every object is spread across every OSD
and losing any given OSD affects all objects. I suppose the severity
depends on the relative fraction of your erasure coding parameters
relative to the total number of OSDs. I think this is perhaps what Erik
was getting at.
Cheers
One rather obvious difference between the two configurations is that the
latter requires a cluster with at least 10 OSD's to make sense. But
let's say we have such a cluster, which of the two configurations would
be recommended, and why?
Thanks,
Erik.
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