Re: Question on IO consideration

Thanks a lot.

So if I get it correct , without spending much time on finding object/segment to move to inmemory. If we just start by setting inmemory_size as 0 (which we already have) and additionally setting inmemory_force=cellmemory should be enough to get started and see the effect of columnar data store in flash. And oracle will automatically put the eligible ones in flash memory and maintain them accordingly as per inbuild algorithm. Thank You.

Regards
Pap

On Wed, Sep 22, 2021 at 9:20 PM Ghassan Salem <salem.ghassan_at_gmail.com> wrote:

> cellmemory does not need the table to be inmemory. It is automatic, but
> can be enforced/inhibited by using the cellmemory attribute for a segment.
> the inmemory area is not in the buffer cache, but a separate area in the
> sga.
> cellmemory is managed by the storage software, and there are limits on the
> space it takes in the flash, so that it does not affect other types of
> usage (e.g. oltp operations,....)
> Also, the format of cellmemory is different from that of columnar
> flashcache, the later uses the HCC format, while cellmemory uses the
> inmemory format, and also enables usage of the same SIMD and other
> algorithms implemented in the inmemory option on db nodes.
>
> I don't think that there's an easy way to see what tables need to go
> there. Best way is to enable it and see the results. If you ask your Sales
> Rep for authorization to do so, say for a month, he'll probably accept. If
> you have an ExaCS or ExaCC, then it is inclmuded and you can try it.
>
> On Wed, Sep 22, 2021 at 4:19 PM Pap <oracle.developer35_at_gmail.com> wrote:
>
>> Great Learning.
>>
>> Lok, So i think what Ghassan is trying to point out that, previously(Pre
>> 18.1 cell server version) HCC enabled tables/partitions can only be
>> stored as columnar format in flash cache , but in recent versions even non
>> HCC tables can be stored in columnar format in flash cache but it has to be
>> inmemory option enabled (i.e. Create table/alter table TAB1 inmemory..
>> etc). And the question on why you don't see statistics ' 'cell physical IO
>> bytes saved by columnar cache' even with HCC enabled table/partitions, i
>> believe the DB version has to be at least 12.1 for this to work, but yours
>> is 11.2.0.4.
>>
>> Ghassan, I had some related questions , with regards to normal buffer
>> cache if we use inmemory feature and enable that at table level (using
>> alter table .. inmemory..) that will try to store the data in columnar
>> format in a separate memory area(of size as defined by inmemory_size
>> parameter) within buffer cache itself. As you mentioned, just keeping
>> INMEMORY_FORCE=CELLMEMORY_LEVEL, and leaving inmemory_size =0 will be
>> enough for objects to be stored in cellmemory in columnar format. So in
>> this case , 1)Isn't it required at all to define the table/partiton as
>> inmemory in the first place?
>>
>> 2)Is there a risk if we define a big table (say ~30TB+) in size as in
>> memory enabled(inmemory_size =0) , will that be going to create
>> issues/saturate flash or will it be handled automatically by Oracle exadata
>> software? Basically how safe is it to turn on the inmemory feature and play
>> with different tables to pin in/out as inmemory?
>>
>> 3) As Lothar pointed out, how can we query dba_hist_active_sess_history
>> for a sql_ids to see nd interpret if inmemory or columnar format is really
>> going to benefit us by reducing IO usage?
>>
>> Regards
>> Pap
>>
>> On Wed, Sep 22, 2021 at 5:32 PM Ghassan Salem <salem.ghassan_at_gmail.com>
>> wrote:
>>
>>> Lok,
>>> As I said, columnar flashcache requires tables to be HCC compressed. As
>>> you're using version 19 of the storage software, that should be ok. For the
>>> inmemory on flashcache (cellmemory), you'll need inmemory option, plus 19c
>>> DB.Staring 19.8, you can set INMEMORY_FORCE=CELLMEMORY_LEVEL, and leave
>>> inmemory_size =0. And yes, inmemory_size has no relation to the amount of
>>> flashcache that can be used by this option.
>>> The first version of this required HCC compressed tables, this was
>>> removed in more recent versions.
>>>
>>> On Wed, Sep 22, 2021 at 12:33 PM Lok P <loknath.73_at_gmail.com> wrote:
>>>
>>>> When you said, *"The first versions of this required HCC tables, but
>>>> the latest ones do not." *Do you mean the HCC table is the minimum
>>>> requirement for columnar flash cache storage in absence of a
>>>> inmemory option. But with the in memory option, it really doesn't matter
>>>> whether it's HCC enabled or not. It can go for columnar flash cache storage
>>>> even HCC not enabled in this case. And the inmemory size parameter
>>>> doesn't have any relation with the amount of flash cache memory used for
>>>> columnar storage by oracle. Correct me if I'm wrong.
>>>>
>>>>
>>>> On Wed, Sep 22, 2021 at 2:12 PM Ghassan Salem <salem.ghassan_at_gmail.com>
>>>> wrote:
>>>>
>>>>> Lok,
>>>>> You'll see columnar flashcache if you have tables that use HCC. And
>>>>> HCC is not automatic, you have to tell the DB to do it.
>>>>>
>>>>> As for the in-memory flashcache, it requires that you have the
>>>>> in-memory option, and that you just set inmemory_size to an acceptable
>>>>> value (depending on your sga...). This will let the storage software use
>>>>> the inmemory format and libraries but using the flashcache instead of the
>>>>> db node ram. There's no need for you to modify the tables to tell it to do
>>>>> that. The first versions of this required HCC tables, but the latest ones
>>>>> do not.
>>>>>
>>>>> So these features depend on the db version, as well as storage
>>>>> software version.
>>>>>
>>>>>
>>>>> On Wed, Sep 22, 2021 at 10:19 AM Lok P <loknath.73_at_gmail.com> wrote:
>>>>>
>>>>>> Thank You All. I was trying to query a few databases. And the below
>>>>>> query gives Zero for most. But I checked all of them having the cell server
>>>>>> version is 19.2+. Few of the databases are in 11.2 though. We do have big
>>>>>> partition tables with compressed historical partitions as 'query high'. So
>>>>>> I wanted to understand, as we are talking mainly three separate things
>>>>>>
>>>>>> 1)Database in memory option which needs additional license and also
>>>>>> it will need inmemory_size parameter to be set as nonzero (which we don't
>>>>>> have set currently). And required a minimum 12.1+ Db version and is
>>>>>> independent of the cell server version. And I think it will also need
>>>>>> specific changes to the table/column definition to make it in memory.
>>>>>> Correct me if I'm wrong.
>>>>>>
>>>>>> 2)Columnar flash cache store format, which I believe is default for
>>>>>> all 12.1+ cell server versions irrespective of database version. And oracle
>>>>>> will do it automatically based on OLTP vs warehousing type load. So why am
>>>>>> I seeing the results out of the below query as zero in some of our
>>>>>> databases? And also 'cell physical IO bytes saved by columnar cache' as
>>>>>> zero. Is there any other parameter that needs to be tweaked to use this
>>>>>> feature?
>>>>>>
>>>>>> 3)And _at_Ghassan , as you mentioned just now, is 'in memory flash
>>>>>> cache' . Is it different from the above two and is dependent on specific DB
>>>>>> and cell server version or parameter change?
>>>>>>
>>>>>> Apology if I am mixing up multiple things and interpreting them wrong.
>>>>>>
>>>>>>
>>>>>> select
>>>>>> xmlcast(xmlquery('/cell_stats/stats/stats/stat[_at_name="columnar_cache_size"]'
>>>>>> passing xmltype(STATISTICS_VALUE) returning content) as varchar2(200) )
>>>>>> "COLUMNAR CACHE SIZE"
>>>>>> from v$cell_state
>>>>>> where xmlexists('/cell_stats/stats/stats[_at_type="columnarcache"]'
>>>>>> passing xmltype(STATISTICS_VALUE));
>>>>>>
>>>>>> On Wed, Sep 22, 2021 at 1:22 PM Ghassan Salem <
>>>>>> salem.ghassan_at_gmail.com> wrote:
>>>>>>
>>>>>>> Mladen,
>>>>>>> The feature Tanel is mentioning is free, not to be confused with
>>>>>>> in-memory flashcache, which requires the inmemory option.
>>>>>>>
>>>>>>> Regards
>>>>>>>
>>>>>>> On Wed, Sep 22, 2021 at 1:59 AM Mladen Gogala <
>>>>>>> gogala.mladen_at_gmail.com> wrote:
>>>>>>>
>>>>>>>>
>>>>>>>> On 9/21/21 17:41, Tanel Poder wrote:
>>>>>>>> > Exadata storage cells (starting from cellsrv v12.1.2.1.0 / Jan
>>>>>>>> 2015)
>>>>>>>> > can use fully columnar flash cache for greatly speeding up reads
>>>>>>>> (the
>>>>>>>> > cache is fully columnar, not hybrid like the datafile storage).
>>>>>>>>
>>>>>>>>
>>>>>>>> And all they want in exchange for that capability is your
>>>>>>>> first-born.
>>>>>>>> Bargain price, indeed. BTW, MariaDB can also do that, a bit cheaper.
>>>>>>>>
>>>>>>>> --
>>>>>>>> Mladen Gogala
>>>>>>>> Database Consultant
>>>>>>>> Tel: (347) 321-1217
>>>>>>>> https://dbwhisperer.wordpress.com
>>>>>>>>
>>>>>>>> --
>>>>>>>> http://www.freelists.org/webpage/oracle-l
>>>>>>>>
>>>>>>>>
>>>>>>>>

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