On Wed, May 4, 2022 at 1:25 AM Eneko Lacunza <elacunza@xxxxxxxxx> wrote: > Hi Gregory, > > El 3/5/22 a las 22:30, Gregory Farnum escribió: > > On Mon, Apr 25, 2022 at 12:57 AM Eneko Lacunza <elacunza@xxxxxxxxx> <elacunza@xxxxxxxxx> wrote: > > We're looking to deploy a stretch cluster for a 2-CPD deployment. I have > read the following docs:https://docs.ceph.com/en/latest/rados/operations/stretch-mode/#stretch-clusters > > I have some questions: > > - Can we have multiple pools in a stretch cluster? > > Yes. > > > - Can we have multiple different crush rules in a stretch cluster? I'm > asking this because the command for stretch mode activation asks for a > rule... > > Right, so what happens there is any pool with a default rule gets > switched to the specified CRUSH rule. That doesn't stop you from > changing the rule after engaging stretch mode, or giving it a > non-default rule ahead of time. You just have to be careful to make > sure it satisfies the stretch mode rules about placing across data > centers. > > So the only purpose of the "stretch_rule" param to "enable_stretch_mode" > is replacing default replicated rule? > > If so, when stretch mode is activated, the described special behaviour > applies to all crush rules/pools: > > - the OSDs will only take PGs active when they peer across data centers > (or whatever other CRUSH bucket type you specified), assuming both are alive > > - Pools will increase in size from the default 3 to 4, expecting 2 copies > in each site (for size=2 pools will it increase to 4 too?) > > Is this accurate? > Yep! AFAIK most clusters just use the default pool for replication, barring mixed media types. *shrug* > > We want to have different purpose pools on this Ceph cluster: > > - Important VM disks, with 2 copies in each DC (SSD class) > - Ephemeral VM disks, with just 2 copies overall (SSD class) > - Backup data in just one DC (HDD class). > > Objective of the 2-DC deployment is disaster recovery, HA isn't > required, but I'll take it if deployment is reasonable :-) . > > I'm leery of this for the reasons described in the docs — if you don't > have 2 replicas per site, you lose data availability every time an OSD > goes down for any reason (or else you have a window while recovery > happens where the data is not physically available in both sites, > which rather negates the purpose). > > > Is this because the following: "the OSDs will only take PGs active when > they peer across data centers (or whatever other CRUSH bucket type you > specified), assuming both are alive"? > > > This conclusion wasn't obvious to me, but after your reply and a third > read, now seems expected :-) > Right! > > Thanks for your comments. I'll be doing some tests, will write back if I > find something unexpected (to me?) :-) > > Cheers > > Eneko Lacunza > Zuzendari teknikoa | Director técnico > Binovo IT Human Project > > Tel. +34 943 569 206 | https://www.binovo.esAstigarragako Bidea, 2 - 2º <https://www.google.com/maps/search/Astigarragako+Bidea,+2+-+2%C2%BA?entry=gmail&source=g> izda. Oficina 10-11, 20180 Oiartzun > https://www.youtube.com/user/CANALBINOVOhttps://www.linkedin.com/company/37269706/ > > _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
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