On Sun, Oct 25, 2020 at 12:11 PM huw...@outlook.com <huw...@outlook.com> wrote:
>
> Hi all,
>
> We are planning for a new pool to store our dataset using CephFS. These data
> are almost read-only (but not guaranteed) and consist of a lot of small
> files. Each node in our cluster has 1 * 1T SSD and 2 * 6T HDD, and we will
> deploy about 10 such nodes. We aim at getting the highest read throughput.
>
> If we just use a replicated pool of size 3 on SSD, we should get the best
> performance, however, that only leave us 1/3 of usable SSD space. And EC
> pools are not friendly to such small object read workload, I think.
>
> Now I’m evaluating a mixed SSD and HDD replication strategy. Ideally, I want
> 3 data replications, each on a different host (fail domain). 1 of them on
> SSD, the other 2 on HDD. And normally every read request is directed to SSD.
> So, if every SSD OSD is up, I’d expect the same read throughout as the all
> SSD deployment.
>
> I’ve read the documents and did some tests. Here is the crush rule I’m
> testing with:
>
> rule mixed_replicated_rule {
> id 3
> type replicated
> min_size 1
> max_size 10
> step take default class ssd
> step chooseleaf firstn 1 type host
> step emit
> step take default class hdd
> step chooseleaf firstn -1 type host
> step emit
> }
>
> Now I have the following conclusions, but I’m not very sure:
> * The first OSD produced by crush will be the primary OSD (at least if I
> don’t change the “primary affinity”). So, the above rule is guaranteed to map
> SSD OSD as primary in pg. And every read request will read from SSD if it is
> up.
> * It is currently not possible to enforce SSD and HDD OSD to be chosen from
> different hosts. So, if I want to ensure data availability even if 2 hosts
> fail, I need to choose 1 SSD and 3 HDD OSD. That means setting the
> replication size to 4, instead of the ideal value 3, on the pool using the
> above crush rule.
>
> Am I correct about the above statements? How would this work from your
> experience? Thanks.
This works (i.e. guards against host failures) only if you have
strictly separate sets of hosts that have SSDs and that have HDDs.
I.e., there should be no host that has both, otherwise there is a
chance that one hdd and one ssd from that host will be picked.
--
Alexander E. Patrakov
CV: http://pc.cd/PLz7
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