Re: Supporting localise Read for RGW

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It would probably make sense to add support in the objecter for
picking up certain flags (such as LOCALIZE_READS) from the pool
config.
-Sam

On Sun, Jun 18, 2023 at 1:34 AM Josh Salomon <jsalomon@xxxxxxxxxx> wrote:
>
> One more point - from a business perspective, this flag is per pool and not global so the API should be per pool and not global.
> Regards,
>
> Josh
>
>
> On Sat, Jun 17, 2023 at 4:22 PM Ilya Dryomov <idryomov@xxxxxxxxx> wrote:
>>
>> On Fri, Jun 16, 2023 at 5:13 PM Casey Bodley <cbodley@xxxxxxxxxx> wrote:
>> >
>> > On Fri, Jun 16, 2023 at 3:21 AM Jiffin Thottan <jthottan@xxxxxxxxxx> wrote:
>> > >
>> > > Hi,
>> > >
>> > > I am planning to support read localise feature for RGW servers similar to what RBD volumes support. From code reading, it looks to pass "librados::OPERATION_LOCALIZE_READS" before sending the request to RADOS.  I have created a tracker issue for this feature https://tracker.ceph.com/issues/61701, this will be per server config option. I don't know about any other technical hurdles in implementing this feature. Please share your thoughts on the same.
>> > >
>> > > Thanks and regards,
>> > > Jiffin
>> >
>> > thanks Jiffin,
>> >
>> > that sounds simple enough, though rgw issues librados reads in many
>> > different places that would need to manage this flag
>> >
>> > i'm curious why this is a per-op flag, rather than a global librados
>> > setting. is there a reason that rgw would only want *some* of its
>> > reads localized? are there any implications for read-after-write
>> > consistency that we need to worry about here?
>>
>> Hi Casey,
>>
>> I'm not aware of any read-after-write or other consistency
>> implications.  Read-from-replica is safe for general use since Octopus
>> (OSD-side issues were fixed in [1]).
>>
>> Ultimately all flags are per-op.  The reason that there is no global
>> librados setting for BALANCE_READS and LOCALIZE_READS seems to be
>> a combination of a) the fact that these flags could only be used in
>> special circumstances in the past and b) the fact that librados doesn't
>> have a generic API for applying a given flag globally like it does for
>> applying a given snapshot ID/context to all reads/writes.
>>
>> [1] https://github.com/ceph/ceph/pull/32381
>>
>> Thanks,
>>
>>                 Ilya
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