ceph osd erasure-code-profile set lrc1 plugin=lrc k=9 m=3 l=4 crush-failure-domain=host crush-locality=room crush-device-class=hdd
I created a pool with only one PG to make the output more readable.This profile should allow the cluster to sustain the loss of three chunks, the results are interesting. This is what I tested:
1. I stopped all OSDs on one host and the PG was still active with one missing chunk, everything's good. 2. Stopping a second host in the same DC resulted in the PG being marked as "down". That was unexpected since with m=3 I expected the PG to still be active but degraded. Before test #3 I started all OSDs to have the PG active+clean again. 3. I stopped one host per DC, so in total 3 chunks were missing and the PG was still active.
Apparently, this profile is able to sustain the loss of m chunks, but not an entire DC. I get the impression (and I also discussed this with a colleague) that LRC with this implementation is either designed to loose only single OSDs which can be recovered quicker with fewer surviving OSDs and saving bandwidth. Or this is a bug because according to the low-level description [1] the algorithm works its way up in the reverse order within the configured layers, like in this example (not displaying my k, m, l requirements, just for reference):
chunk nr 01234567 step 1 _cDD_cDD step 2 cDDD____ step 3 ____cDDDSo if a whole DC fails and the chunks from step 3 can not be recovered, and maybe step 2 also fails, but eventually step 1 contains the actual k and m chunks which should sustain the loss of an entire DC. My impression is that the algorithm somehow doesn't arrive at step 1 and therefore the PG stays down although there are enough surviving chunks. I'm not sure if my observations and conclusion are correct, I'd love to have a comment from the developers on this topic. But in this state I would not recommend to use the LRC plugin when the resiliency requirements are to sustain the loss of an entire DC.
Thanks, Eugen[1] https://docs.ceph.com/en/latest/rados/operations/erasure-code-lrc/#low-level-plugin-configuration
Zitat von Michel Jouvin <michel.jouvin@xxxxxxxxxxxxxxx>:
Hi,I realize that the crushmap I attached to one of my email, probably required to understand the discussion here, has been stripped down by mailman. To avoid poluting the thread with a long output, I put it on at https://box.in2p3.fr/index.php/s/J4fcm7orfNE87CX. Download it if you are interested.Best regards, Michel Le 21/05/2023 à 16:07, Michel Jouvin a écrit :Hi Eugen,My LRC pool is also somewhat experimental so nothing really urgent. If you manage to do some tests that help me to understand the problem I remain interested. I propose to keep this thread for that.Zitat, I shared my crush map in the email you answered if the attachment was not suppressed by mailman.Cheers, Michel Sent from my mobile Le 18 mai 2023 11:19:35 Eugen Block <eblock@xxxxxx> a écrit :Hi, I don’t have a good explanation for this yet, but I’ll soon get the opportunity to play around with a decommissioned cluster. I’ll try to get a better understanding of the LRC plugin, but it might take some time, especially since my vacation is coming up. :-) I have some thoughts about the down PGs with failure domain OSD, but I don’t have anything to confirm it yet. Zitat von Curt <lightspd@xxxxxxxxx>:Hi,I've been following this thread with interest as it seems like a unique usecase to expand my knowledge. I don't use LRC or anything outside basic erasure coding. What is your current crush steps rule? I know you made changes since your first post and had some thoughts I wanted to share, but wanted to see your rule first so I could try to visualize the distribution better. The onlyway I can currently visualize it working is with more servers, I'm thinking6 or 9 per data center min, but that could be my lack of knowledge on some of the step rules. Thanks Curt On Tue, May 16, 2023 at 11:09 AM Michel Jouvin < michel.jouvin@xxxxxxxxxxxxxxx> wrote:Hi Eugen, Yes, sure, no problem to share it. I attach it to this email (as it may clutter the discussion if inline). If somebody on the list has some clue on the LRC plugin, I'm still interested by understand what I'm doing wrong! Cheers, Michel Le 04/05/2023 à 15:07, Eugen Block a écrit :Hi, I don't think you've shared your osd tree yet, could you do that? Apparently nobody else but us reads this thread or nobody reading this uses the LRC plugin. ;-) Thanks, Eugen Zitat von Michel Jouvin <michel.jouvin@xxxxxxxxxxxxxxx>: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@xxxxxx>: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@xxxxxxxxxxxxxxx>: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_sizemin_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@xxxxxxxxxxxxxxx> 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 expectingthe > 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). Andit 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/expertisewith >>>> 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 interms >>>> 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@xxxxxxx >>>> To unsubscribe send an email to ceph-users-leave@xxxxxxx _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx_______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx_______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx_______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx_______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx_______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx_______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx_______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
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