Hi, I am resurrecting this old thread that I started 18 months ago after some new tests. I stopped my initial tests as the cluster I was using had not enough OSD to use 'host' as the failure domain. Thus I was using 'osd' as the failure domain and I understood it was unusual and probably not expected to work... Recently, in another cluster with 3 datacenters and 6 servers (with 18 to 24 OSDs per server) in each datacenter, I gave the LRC plugin another try. And the same happened again after that one of the datacenters went down: all PGs from the EC pool using the LRC plugin went down. I don't really understand the reason but I was wondering if this plugin, which is still documented, is really supported and supposed to work in Reef? If not, I would like to avoid spending too much time troubleshooting it... If somebody is successfully using it, I'm interested to hear it! My erasure code profile definition is: crush-device-class=hdd crush-failure-domain=host crush-locality=datacenter crush-root=default k=9 l=5 m=6 plugin=lrc Best regards, Michel Le 04/05/2023 à 12:51, Michel Jouvin a écrit :
Hi,
I had to restart one of my OSD server today and the problem showed up again. This time I managed to capture "ceph health detail" output showing the problem with the 2 PGs:
[WRN] PG_AVAILABILITY: Reduced data availability: 2 pgs inactive, 2 pgs down pg 56.1 is down, acting [208,65,73,206,197,193,144,155,178,182,183,133,17,NONE,36,NONE,230,NONE] pg 56.12 is down, acting [NONE,236,28,228,218,NONE,215,117,203,213,204,115,136,181,171,162,137,128]
I still doesn't understand why, if I am supposed to survive to a datacenter failure, I cannot survive to 3 OSDs down on the same host, hosting shards for the PG. In the second case it is only 2 OSDs down but I'm surprised they don't seem in the same "group" of OSD (I'd expected all the the OSDs of one datacenter to be in the same groupe of 5 if the order given really reflects the allocation done...
Still interested by some explanation on what I'm doing wrong! Best regards,
Michel
Le 03/05/2023 à 10:21, Eugen Block a écrit :
I think I got it wrong with the locality setting, I'm still limited by the number of hosts I have available in my test cluster, but as far as I got with failure-domain=osd I believe k=6, m=3, l=3 with locality=datacenter could fit your requirement, at least with regards to the recovery bandwidth usage between DCs, but the resiliency would not match your requirement (one DC failure). That profile creates 3 groups of 4 chunks (3 data/coding chunks and one parity chunk) across three DCs, in total 12 chunks. The min_size=7 would not allow an entire DC to go down, I'm afraid, you'd have to reduce it to 6 to allow reads/writes in a disaster scenario. I'm still not sure if I got it right this time, but maybe you're better off without the LRC plugin with the limited number of hosts. Instead you could use the jerasure plugin with a profile like k=4 m=5 allowing an entire DC to fail without losing data access (we have one customer using that).
Zitat von Eugen Block <eblock@nde.ag>:
Hi,
disclaimer: I haven't used LRC in a real setup yet, so there might be some misunderstandings on my side. But I tried to play around with one of my test clusters (Nautilus). Because I'm limited in the number of hosts (6 across 3 virtual DCs) I tried two different profiles with lower numbers to get a feeling for how that works.
# first attempt ceph:~ # ceph osd erasure-code-profile set LRCprofile plugin=lrc k=4 m=2 l=3 crush-failure-domain=host
For every third OSD one parity chunk is added, so 2 more chunks to store ==> 8 chunks in total. Since my failure-domain is host and I only have 6 I get incomplete PGs.
# second attempt ceph:~ # ceph osd erasure-code-profile set LRCprofile plugin=lrc k=2 m=2 l=2 crush-failure-domain=host
This gives me 6 chunks in total to store across 6 hosts which works:
ceph:~ # ceph pg ls-by-pool lrcpool PG OBJECTS DEGRADED MISPLACED UNFOUND BYTES OMAP_BYTES* OMAP_KEYS* LOG STATE SINCE VERSION REPORTED UP ACTING SCRUB_STAMP DEEP_SCRUB_STAMP 50.0 1 0 0 0 619 0 0 1 active+clean 72s 18410'1 18415:54 [27,13,0,2,25,7]p27 [27,13,0,2,25,7]p27 2023-05-02 14:53:54.322135 2023-05-02 14:53:54.322135 50.1 0 0 0 0 0 0 0 0 active+clean 6m 0'0 18414:26 [27,33,22,6,13,34]p27 [27,33,22,6,13,34]p27 2023-05-02 14:53:54.322135 2023-05-02 14:53:54.322135 50.2 0 0 0 0 0 0 0 0 active+clean 6m 0'0 18413:25 [1,28,14,4,31,21]p1 [1,28,14,4,31,21]p1 2023-05-02 14:53:54.322135 2023-05-02 14:53:54.322135 50.3 0 0 0 0 0 0 0 0 active+clean 6m 0'0 18413:24 [8,16,26,33,7,25]p8 [8,16,26,33,7,25]p8 2023-05-02 14:53:54.322135 2023-05-02 14:53:54.322135
After stopping all OSDs on one host I was still able to read and write into the pool, but after stopping a second host one PG from that pool went "down". That I don't fully understand yet, but I just started to look into it. With your setup (12 hosts) I would recommend to not utilize all of them so you have capacity to recover, let's say one "spare" host per DC, leaving 9 hosts in total. A profile with k=3 m=3 l=2 could make sense here, resulting in 9 total chunks (one more parity chunks for every other OSD), min_size 4. But as I wrote, it probably doesn't have the resiliency for a DC failure, so that needs some further investigation.
Regards, Eugen
Zitat von Michel Jouvin <michel.jouvin@ijclab.in2p3.fr>:
Hi,
No... our current setup is 3 datacenters with the same configuration, i.e. 1 mon/mgr + 4 OSD servers with 16 OSDs each. Thus the total of 12 OSDs servers. As with LRC plugin, k+m must be a multiple of l, I found that k=9/m=66/l=5 with crush-locality=datacenter was achieving my goal of being resilient to a datacenter failure. Because I had this, I considered that lowering the crush failure domain to osd was not a major issue in my case (as it would not be worst than a datacenter failure if all the shards are on the same server in a datacenter) and was working around the lack of hosts for k=9/m=6 (15 OSDs).
May be it helps, if I give the erasure code profile used:
crush-device-class=hdd crush-failure-domain=osd crush-locality=datacenter crush-root=default k=9 l=5 m=6 plugin=lrc
The previously mentioned strange number for min_size for the pool created with this profile has vanished after Quincy upgrade as this parameter is no longer in the CRUH map rule! and the `ceph osd pool get` command reports the expected number (10):
---------
ceph osd pool get fink-z1.rgw.buckets.data min_size min_size: 10
Cheers,
Michel
Le 29/04/2023 à 20:36, Curt a écrit :
Hello,
What is your current setup, 1 server pet data center with 12 osd each? What is your current crush rule and LRC crush rule?
On Fri, Apr 28, 2023, 12:29 Michel Jouvin <michel.jouvin@ijclab.in2p3.fr> wrote:
Hi,
I think I found a possible cause of my PG down but still understand why. As explained in a previous mail, I setup a 15-chunk/OSD EC pool (k=9, m=6) but I have only 12 OSD servers in the cluster. To workaround the problem I defined the failure domain as 'osd' with the reasoning that as I was using the LRC plugin, I had the warranty that I could loose a site without impact, thus the possibility to loose 1 OSD server. Am I wrong?
Best regards,
Michel
Le 24/04/2023 à 13:24, Michel Jouvin a écrit : > Hi, > > I'm still interesting by getting feedback from those using the LRC > plugin about the right way to configure it... Last week I upgraded > from Pacific to Quincy (17.2.6) with cephadm which is doing the > upgrade host by host, checking if an OSD is ok to stop before actually > upgrading it. I had the surprise to see 1 or 2 PGs down at some points > in the upgrade (happened not for all OSDs but for every > site/datacenter). Looking at the details with "ceph health detail", I > saw that for these PGs there was 3 OSDs down but I was expecting the > pool to be resilient to 6 OSDs down (5 for R/W access) so I'm > wondering if there is something wrong in our pool configuration (k=9, > m=6, l=5). > > Cheers, > > Michel > > Le 06/04/2023 à 08:51, Michel Jouvin a écrit : >> 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 >>>> _______________________________________________ >>>> ceph-users mailing list -- ceph-users@ceph.io >>>> To unsubscribe send an email to ceph-users-leave@ceph.io _______________________________________________ ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-leave@ceph.io
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