Re: upmap balancer and consequences of osds briefly marked out
Do I misunderstand this script, or does it not _quite_ do what’s desired here? I fully get the scenario of applying a full-cluster map to allow incremental topology changes. To be clear, if this is run to effectively freeze backfill during / following a traumatic event, it will freeze that adapted state, not strictly return to the pre-event state? And thus the pg-upmap balancer would still need to be run to revert to the prior state? And this would also hold true for a failed/replaced OSD?
On May 1, 2020, at 7:37 AM, Dylan McCulloch <dmc@unimelb.edu.au> wrote:
Thanks Dan, that looks like a really neat method & script for a few use-cases. We've actually used several of the scripts in that repo over the years, so, many thanks for sharing.
That method will definitely help in the scenario in which a set of unnecessary pg remaps have been triggered and can be caught early and reverted. I'm still a little concerned about the possibility of, for example, a brief network glitch occurring at night and then waking up to a full unbalanced cluster. Especially with NVMe clusters that can rapidly remap and rebalance (and for which we also have a greater impetus to squeeze out as much available capacity as possible with upmap due to cost per TB). It's just a risk I hadn't previously considered and was wondering if others have either run into it or felt any need to plan around it.
Cheers, Dylan
From: Dan van der Ster <dan@vanderster.com> Sent: Friday, 1 May 2020 5:53 PM To: Dylan McCulloch <dmc@unimelb.edu.au> Cc: ceph-users <ceph-users@ceph.io>
Subject: Re: [ceph-users] upmap balancer and consequences of osds briefly marked out
Hi,
You're correct that all the relevant upmap entries are removed when an OSD is marked out. You can try to use this script which will recreate them and get the cluster back to HEALTH_OK quickly: https://github.com/cernceph/ceph-scripts/blob/master/tools/upmap/upmap-remap...
Cheers, Dan
On Fri, May 1, 2020 at 9:36 AM Dylan McCulloch <dmc@unimelb.edu.au> wrote:
Hi all,
We're using upmap balancer which has made a huge improvement in evenly distributing data on our osds and has provided a substantial increase in usable capacity.
Currently on ceph version: 12.2.13 luminous
We ran into a firewall issue recently which led to a large number of osds being briefly marked 'down' & 'out'. The osds came back 'up' & 'in' after about 25 mins and the cluster was fine but had to perform a significant amount of backfilling/recovery despite
there being no end-user client I/O during that period.
Presumably the large number of remapped pgs and backfills were due to pg_upmap_items being removed from the osdmap when osds were marked out and subsequently those pgs were redistributed using the default crush algorithm. As a result of the brief outage our cluster became significantly imbalanced again with several osds very close to full. Is there any reasonable mitigation for that scenario?
The auto-balancer will not perform optimizations while there are degraded pgs, so it would only start reapplying pg upmap exceptions after initial recovery is complete (at which point capacity may be dangerously reduced). Similarly, as admins, we normally only apply changes when the cluster is in a healthy state, but if the same issue were to occur again would it be advisable to manually apply balancer plans while initial recovery is still taking place?
I guess my concern from this experience is that making use of the capacity gained by using upmap balancer appears to carry some risk. i.e. it's possible for a brief outage to remove those space efficiencies relatively quickly and potentially result in full
osds/cluster before the automatic balancer is able to resume and redistribute pgs using upmap.
Curious whether others have any thoughts or experience regarding this.
Cheers, Dylan _______________________________________________ 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
Right, it would freeze the PGs in place at the time upmap-remapped is run. You need to keep running the upmap balancer afterwards to restore the optimized state. I don't quite understand your question about a failed / replaced osd, but yes it is relevant here. Suppose you have osds 0, 1, 2, and 3 and the osd.1 fails: A hypothetical pg_upmap_items which mapped 0 to 1 *and* 2 to 3 would be removed when osd.1 is marked out. This would result in a PG being remapped and data moved from 3 to 2. [1] So if you run upmap-remapped just afterwards, it would create a new pg_upmap_items mapping 2 to 3 and making that PG active+clean again immediately. And then later when you recreate osd.1, crush would recalculate and then after some iterations of the upmap balancer the original pg_upmap_items would be created. -- Dan [1] this hints at an optimization for the "clean upmaps" functionalities in OSDMap.cc -- if an osd is marked out it could modify the relevant pg_upmap_items' accordingly, rather than remove them completely. On Sun, May 3, 2020 at 10:27 PM Anthony D'Atri <aad@dreamsnake.net> wrote:
Do I misunderstand this script, or does it not _quite_ do what’s desired here?
I fully get the scenario of applying a full-cluster map to allow incremental topology changes.
To be clear, if this is run to effectively freeze backfill during / following a traumatic event, it will freeze that adapted state, not strictly return to the pre-event state? And thus the pg-upmap balancer would still need to be run to revert to the prior state? And this would also hold true for a failed/replaced OSD?
On May 1, 2020, at 7:37 AM, Dylan McCulloch <dmc@unimelb.edu.au> wrote:
Thanks Dan, that looks like a really neat method & script for a few use-cases. We've actually used several of the scripts in that repo over the years, so, many thanks for sharing.
That method will definitely help in the scenario in which a set of unnecessary pg remaps have been triggered and can be caught early and reverted. I'm still a little concerned about the possibility of, for example, a brief network glitch occurring at night and then waking up to a full unbalanced cluster. Especially with NVMe clusters that can rapidly remap and rebalance (and for which we also have a greater impetus to squeeze out as much available capacity as possible with upmap due to cost per TB). It's just a risk I hadn't previously considered and was wondering if others have either run into it or felt any need to plan around it.
Cheers, Dylan
From: Dan van der Ster <dan@vanderster.com> Sent: Friday, 1 May 2020 5:53 PM To: Dylan McCulloch <dmc@unimelb.edu.au> Cc: ceph-users <ceph-users@ceph.io>
Subject: Re: [ceph-users] upmap balancer and consequences of osds briefly marked out
Hi,
You're correct that all the relevant upmap entries are removed when an OSD is marked out. You can try to use this script which will recreate them and get the cluster back to HEALTH_OK quickly: https://github.com/cernceph/ceph-scripts/blob/master/tools/upmap/upmap-remap...
Cheers, Dan
On Fri, May 1, 2020 at 9:36 AM Dylan McCulloch <dmc@unimelb.edu.au> wrote:
Hi all,
We're using upmap balancer which has made a huge improvement in evenly distributing data on our osds and has provided a substantial increase in usable capacity.
Currently on ceph version: 12.2.13 luminous
We ran into a firewall issue recently which led to a large number of osds being briefly marked 'down' & 'out'. The osds came back 'up' & 'in' after about 25 mins and the cluster was fine but had to perform a significant amount of backfilling/recovery despite
there being no end-user client I/O during that period.
Presumably the large number of remapped pgs and backfills were due to pg_upmap_items being removed from the osdmap when osds were marked out and subsequently those pgs were redistributed using the default crush algorithm. As a result of the brief outage our cluster became significantly imbalanced again with several osds very close to full. Is there any reasonable mitigation for that scenario?
The auto-balancer will not perform optimizations while there are degraded pgs, so it would only start reapplying pg upmap exceptions after initial recovery is complete (at which point capacity may be dangerously reduced). Similarly, as admins, we normally only apply changes when the cluster is in a healthy state, but if the same issue were to occur again would it be advisable to manually apply balancer plans while initial recovery is still taking place?
I guess my concern from this experience is that making use of the capacity gained by using upmap balancer appears to carry some risk. i.e. it's possible for a brief outage to remove those space efficiencies relatively quickly and potentially result in full
osds/cluster before the automatic balancer is able to resume and redistribute pgs using upmap.
Curious whether others have any thoughts or experience regarding this.
Cheers, Dylan _______________________________________________ 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
participants (2)
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Anthony D'Atri
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Dan van der Ster