Gluster users - Nov 2018 - On making ctime generator enabled by default in stack

On Mon, Nov 5, 2018 at 7:56 PM Raghavendra Gowdappa <rgowdapp at
redhat.com>
wrote:
> All,
>
> There is a patch [1] from Kotresh, which makes ctime generator as default
> in stack. Currently ctime generator is being recommended only for usecases
> where ctime is important (like for Elasticsearch). However, a reliable
> (c)(m)time can fix many consistency issues within glusterfs stack too.
> These are issues with caching layers having stale (meta)data [2][3][4].
> Basically just like applications, components within glusterfs stack too
> need a time to find out which among racing ops (like write, stat, etc) has
> latest (meta)data.
>
> Also note that a consistent (c)(m)time is not an optional feature, but
> instead forms the core of the infrastructure. So, I am proposing to merge
> this patch. If you've any objections, please voice out before Nov 13,
2018
> (a week from today).
>
> As to the existing known issues/limitations with ctime generator, my
> conversations with Kotresh, revealed following:
> * Potential performance degradation (we don't yet have data to
> conclusively prove it, preliminary basic tests from Kotresh didn't
indicate
> a significant perf drop).
>
Do we have this data captured somewhere? If not, would it be possible to
share that data here?

Thanks,
Vijay
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On Tue, Nov 6, 2018 at 9:58 AM Vijay Bellur <vbellur at redhat.com> wrote:
>
>
> On Mon, Nov 5, 2018 at 7:56 PM Raghavendra Gowdappa <rgowdapp at
redhat.com>
> wrote:
>
>> All,
>>
>> There is a patch [1] from Kotresh, which makes ctime generator as
default
>> in stack. Currently ctime generator is being recommended only for
usecases
>> where ctime is important (like for Elasticsearch). However, a reliable
>> (c)(m)time can fix many consistency issues within glusterfs stack too.
>> These are issues with caching layers having stale (meta)data [2][3][4].
>> Basically just like applications, components within glusterfs stack too
>> need a time to find out which among racing ops (like write, stat, etc)
has
>> latest (meta)data.
>>
>> Also note that a consistent (c)(m)time is not an optional feature, but
>> instead forms the core of the infrastructure. So, I am proposing to
merge
>> this patch. If you've any objections, please voice out before Nov
13, 2018
>> (a week from today).
>>
>> As to the existing known issues/limitations with ctime generator, my
>> conversations with Kotresh, revealed following:
>> * Potential performance degradation (we don't yet have data to
>> conclusively prove it, preliminary basic tests from Kotresh didn't
indicate
>> a significant perf drop).
>>
>
> Do we have this data captured somewhere? If not, would it be possible to
> share that data here?
>
I misquoted Kotresh. He had measured impact of gfid2path and said both
features might've similar impact as major perf cost is related to storing
xattrs on backend fs. I am in the process of getting a fresh set of
numbers. Will post those numbers when available.

>
> Thanks,
> Vijay
>
>
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