Hi,
Is somebody using LRC plugin ?
I came to the conclusion that LRC k=9, m=3, l=4 is not the same as
jerasure k=9, m=6 in terms of protection against failures and that I
should use k=9, m=6, l=5 to get a level of resilience >= jerasure k=9,
m=6. The example in the documentation (k=4, m=2, l=3) suggests that this
LRC configuration gives something better than jerasure k=4, m=2 as it is
resilient to 3 drive failures (but not 4 if I understood properly). So
how many drives can fail in the k=9, m=6, l=5 configuration first
without loosing RW access and second without loosing data?
Another thing that I don't quite understand is that a pool created with
this configuration (and failure domain=osd, locality=datacenter) has a
min_size=3 (max_size=18 as expected). It seems wrong to me, I'd expected
something ~10 (depending on answer to the previous question)...
Thanks in advance if somebody could provide some sort of authoritative
answer on these 2 questions. Best regards,
Michel
Le 04/04/2023 à 15:53, Michel Jouvin a écrit :
Answering to myself, I found the reason for 2147483647: it's
documented as a failure to find enough OSD (missing OSDs). And it is
normal as I selected different hosts for the 15 OSDs but I have only
12 hosts!
I'm still interested by an "expert" to confirm that LRC k=9, m=3, l=4
configuration is equivalent, in terms of redundancy, to a jerasure
configuration with k=9, m=6.
Michel
Le 04/04/2023 à 15:26, Michel Jouvin a écrit :
Hi,
As discussed in another thread (Crushmap rule for multi-datacenter
erasure coding), I'm trying to create an EC pool spanning 3
datacenters (datacenters are present in the crushmap), with the
objective to be resilient to 1 DC down, at least keeping the readonly
access to the pool and if possible the read-write access, and have a
storage efficiency better than 3 replica (let say a storage overhead
<= 2).
In the discussion, somebody mentioned LRC plugin as a possible
jerasure alternative to implement this without tweaking the crushmap
rule to implement the 2-step OSD allocation. I looked at the
documentation
(https://docs.ceph.com/en/latest/rados/operations/erasure-code-lrc/)
but I have some questions if someone has experience/expertise with
this LRC plugin.
I tried to create a rule for using 5 OSDs per datacenter (15 in
total), with 3 (9 in total) being data chunks and others being coding
chunks. For this, based of my understanding of examples, I used k=9,
m=3, l=4. Is it right? Is this configuration equivalent, in terms of
redundancy, to a jerasure configuration with k=9, m=6?
The resulting rule, which looks correct to me, is:
--------
{
"rule_id": 6,
"rule_name": "test_lrc_2",
"ruleset": 6,
"type": 3,
"min_size": 3,
"max_size": 15,
"steps": [
{
"op": "set_chooseleaf_tries",
"num": 5
},
{
"op": "set_choose_tries",
"num": 100
},
{
"op": "take",
"item": -4,
"item_name": "default~hdd"
},
{
"op": "choose_indep",
"num": 3,
"type": "datacenter"
},
{
"op": "chooseleaf_indep",
"num": 5,
"type": "host"
},
{
"op": "emit"
}
]
}
------------
Unfortunately, it doesn't work as expected: a pool created with this
rule ends up with its pages active+undersize, which is unexpected for
me. Looking at 'ceph health detail` output, I see for each page
something like:
pg 52.14 is stuck undersized for 27m, current state
active+undersized, last acting
[90,113,2147483647,103,64,147,164,177,2147483647,133,58,28,8,32,2147483647]
For each PG, there is 3 '2147483647' entries and I guess it is the
reason of the problem. What are these entries about? Clearly it is
not OSD entries... Looks like a negative number, -1, which in terms
of crushmap ID is the crushmap root (named "default" in our
configuration). Any trivial mistake I would have made?
Thanks in advance for any help or for sharing any successful
configuration?
Best regards,
Michel
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