Hi All, I found in many articles(two of the samples is below) that the
smart scan wont use the flash cache by default. But in our case, we are
seeing a query doing the smart scan using flash cache heavily and making it
reach flash IOPS to ~200K and response time around 15-20ms. Are there any
bugs around that which we are hitting? or is there any other setting
driving this or my understanding is wrong here?
Actually in our case it's exadata X5 machine with image version 19.2, half
RAC with ~40TB of flash cache and its hosting single database on it which
is on version 11.2.0.4. One of the query which does cell smart scan on a
big partitioned table(TAB1 , daily range partitioned on column PART_DT) and
it scans through all the ~500+ partition(which is as per the business
requirement), the flash disk utilization went up to ~80% reaching ~200K
IOPS with response time of ~15-20ms. And at the same time the hard disk
utilization and IOPS stays below ~40%. The flash disk is showing high large
reads during that interval and so index read/small reads are getting
impacted. I verified the FLASH_CACHE and DEFAULT_FLASH_CACHE values in
dba_tab_partitions both are DEFAULT for this object. Want to understand why
it's happening that way?
https://www.informit.com/articles/article.aspx?p=2418151&seqNum=3
http://guyharrison.squarespace.com/blog/2013/12/30/can-the-exadata-smart-flash-cache-slow-smart-scans.html
Regards
Lok
