On Tue, Feb 22, 2022 at 1:19 PM 郭 明 <guoracle@xxxxxxxxxxx> wrote:
>
> Hi Ilya,
>
> Thanks for your reply and valuable insights.
>
> If the data in the EC pool is not modified after being written, and only supports reading, can the replica OSD support BALANCE_READS or LOCALIZE_READS by acting as the primary OSD (same as the primary OSD, reading multiple data chunks, just sharing the pressure of the primary OSD) ?
In theory, yes -- something like that could be implemented but the
"not modified after being written" part is a pretty big assumption
and I don't know how complicated the implementation would be.
Thanks,
Ilya
>
>
> All the best，
> guoming
> ________________________________
> 发件人: Ilya Dryomov <idryomov@xxxxxxxxx>
> 发送时间: 2022年2月21日 17:21
> 收件人: 郭 明 <guoracle@xxxxxxxxxxx>
> 抄送: dev <dev@xxxxxxx>; Samuel Just <sjust@xxxxxxxxxx>
> 主题: Re: need your help on a ceph pr
>
> On Mon, Feb 21, 2022 at 12:12 PM 郭 明 <guoracle@xxxxxxxxxxx> wrote:
> >
> > Hello Dryomov，
> > How is your life going?
> > I have some questions about the follow pr:
> > https://github.com/ceph/ceph/pull/35326
> > Objecter: don't attempt to read from non-primary on EC pools by idryomov · Pull Request #35326 · ceph/ceph
> > With BALANCE_READS or LOCALIZE_READS set, the client will hang if the non-primary OSD is picked because the OSD will most likely drop the op (or start waiting for peering that won't actually happen...
> > github.com
> >
> >
> > 1). Could you tell me more details of the reason about "the OSD will most likely drop
> > the op (or start waiting for peering that won't actually happen)" ?
>
> Hi guoming,
>
> In the EC pool case, all OSDs in the PG have different data. If
> a read is directed at a non-primary OSD, it simply may not have the
> data available. And if it turns out to have the required EC chunk,
> servicing a read with that data would most likely be unsafe.
>
> >
> > 2).Why only EC pool has this problem, and replicated pool does not have this problem?
>
> In the replicated pool case, all OSDs in the PG have the same data, so
> theoretically any OSD has the ability to service any read. But even in
> the replicated case there are safety issues. Prior to this [1] change
> by Sam, BALANCE_READS and LOCALIZE_READS flags were unsafe for general
> use and after it there are still a couple of cases when the OSD either
> drops the op or returns EAGAIN, expecting the client to resend it to
> the primary because otherwise wrong data could be returned to the
> client.
>
> I'm adding Sam for more details.
>
> >
> > 3).Is this solution unique? Can this problem be solved by modifying the OSD module?
>
> If you are asking whether the OSD can be modified to support
> BALANCE_READS or LOCALIZE_READS on EC pools, the answer is no
> because each OSD stores its own EC chunk.
>
> [1] https://github.com/ceph/ceph/pull/32381
>
> Thanks,
>
> Ilya
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