Re: Higher Read response in 19C

If it's just one odd query impacting response time significantly, you may try setting work area size manual with hash area size for that specific session to minimize temp spill.

On Sat, 17 Sep 2022, 7:57 pm Lok P, <loknath.73_at_gmail.com> wrote:

> I see from the below IO stats summary, it says the tempfile read in case
> of 11g for the same 4hrs duration of AWR the small read response ~1.14ms vs
> in case of 19c its 26.84ms. So is there any different handling of temp
> reads in 11.2 vs 19c? Or is it just that the small read must be served from
> hard disk rather than flash disk because of some other reason or the way
> now 19c deals with the same underlying X8M storage cells?
>
> *On 11G:-*
>
> IOStat by Filetype summary
> 'Data' columns suffixed with M,G,T,P are in multiples of 1024 other
> columns suffixed with K,M,G,T,P are in multiples of 1000
> Small Read and Large Read are average service times
> Ordered by (Data Read + Write) desc
> Filetype Name Reads: Data Reqs per sec Data per sec Writes: Data Reqs per
> sec Data per sec Small Read Large Read
> Data File 11.6T 4532.3 1.1G 800G 1773.51 75.922M 346.18us 897.65ms
> Temp File 1.3T 1007.67 122.029M 276.4G 195.14 26.234M 1.14ms 43.33ms
> Log File 603.3G 57.35 57.253M 901.5G 609.87 85.558M 2.04ms 45.14ms
> Archive Log 0M 0 0M 463.5G 43.99 43.985M
> Control File 18.4G 94.96 1.745M 1.1G 6.58 .103M 306.61us 3.38ms
> Other 31M 0.78 .003M 0M 0 0M 1.88ms
> TOTAL: 13.5T 5693.05 1.3G 2.4T 2629.09 231.803M 526.09us 839.93ms
>
>
> *On 19C:*
> IOStat by Filetype summary
> 'Data' columns suffixed with M,G,T,P are in multiples of 1024 other
> columns suffixed with K,M,G,T,P are in multiples of 1000
> Small Read and Large Read are average service times
> Ordered by (Data Read + Write) desc
> Filetype Name Reads: Data Reqs per sec Data per sec Writes: Data Reqs per
> sec Data per sec Small Read Large Read
> Data File 107.4T 15779.97 10.2G 1009.6G 3532.94 95.607M 718.98us 347.66ms
> Log File 499.5G 47.61 47.301M 996.5G 261.48 94.365M 2.78ms 59.84ms
> Archive Log 0M 0 0M 493.9G 46.77 46.771M
> Temp File 190.3G 37.4 18.017M 294.1G 56.65 27.853M 26.84ms 39.44ms
> Control File 20.8G 102.93 1.972M 475M 2.81 .044M 619.77us 6.33ms
> TOTAL: 108.1T 15967.91 10.2G 2.7T 3900.65 264.641M 763.90us 345.57ms
>
>
>
>
>
> On Sat, Sep 17, 2022 at 3:31 PM Lok P <loknath.73_at_gmail.com> wrote:
>
>> Hello Listers, Post 19C migration we are seeing the response time of
>> 'direct path read temp' has been bumped up from ~1.6ms to ~36ms. So we have
>> many queries doing temp read while performing hash join performing slow.
>> Also we have observed the amount of tempspill remains the same as it was
>> before for those queries. We had a PGA size 40GB of PGA on 11g , on 19c we
>> made it 60GB. Wanted to understand the cause of this high temp read
>> response and how to fix.
>>
>> Attached are the Key instance activity, top foreground waitevent and
>> instance efficiency , sections from AWR from a similar load for 11g vs 19c.Its
>> X8M half RAC exadata machine with 19.3.7 image. And in this migration no
>> storage cell level changes were done; it was only the RDBMS migration from
>> version 11.2.0.4 to 19.15. Looking into the AWR for the similar load
>> period from 11g to 19c, I am seeing below..
>>
>> 1)In the "Instance Efficiency Percentages" section it shows the "Flash
>> Cache Hit %" for the similar load period was 25% on 11g vs ~47% on 19C.
>>
>> 2)The "Top 10 Foreground Events by Total Wait Time" section showing there
>> were ~21million "cell single block physical read" and avg wait time was
>> ~621 micro sec on 11g. and for "direct path read temp" total waits were
>> ~3.5million with ~1.68milli sec avg wait time.
>>
>> But on 19c awr its showing four different stats for "cell single block
>> physical read" i.e.
>>
>> "cell single block physical read: flash cache" showing ~2.5million waits
>> and ~6.1ms avg wait time.
>>
>> "cell single block physical read: pmem cache" showing ~10million waits
>> and ~1.16ms avg wait time.
>>
>> "cell single block physical read" showing 104K waits and ~47.37ms avg
>> wait time .
>>
>> "cell single block physical read: RDMA" showing 33million waits and
>> 69.56micro sec avg wait time.
>>
>> "direct path read temp" showing 373K waits and 36.04 milli sec avg wait
>> time.
>>
>> 3)"IOStat by Filetype summary" on 11g was showing ~1.3TB data read with
>> "small read" response ~1.13ms and "large read" 43.33ms. But on 19c the read
>> is ~190.3GB with "small read" response ~26.84ms and "large read" 39.44ms.
>> It does point to the slow temp file read.
>>
>> 4) We do have the cell storage flash cache mode "write back" mode but i
>> am seeing "PMEMCACHE- writethrough" so not sure if its something adding
>> to this issue?
>>
>> 5) In the "cache sizes" section I am seeing an additional NUMA Pool
>> Size. Not sure if it's related.
>>
>> Thanks and Regards
>>
>> Lok
>>
>>
>>

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