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DMLs and the Columnar Cache on ADW

Posted on August 23, 2019 by Roger MacNicol Posted in 12.2, inmemory, oracle Leave a comment

One of the main performance technologies underlying ADW is the In-Memory format column cache and a question that has come up several times is: how does the columnar cache handle DMLs and cache invalidations. This blog post attempts to answer that question.

The Two Columnar Cache Formats

The original columnar cache was an idempotent rearrangement of Hybrid Columnar Compressed blocks into pure columnar form still in the original HCC encoding. This is known internally as “CC1” and was not applicable to row format blocks. Because this is an idempotent transformation we are able to reconstitute the original raw blocks from a CC1 cache if needed.

We then introduced the In-Memory format columnar cache where, if you had an In-Memory licence, we would run the 1 MB chunks processed by Smart Scan through the In-Memory loader and write new clean columns encoded in In-Memory formats which meant that we could then use the new SIMD based predicate evaluation and other performance improvements developed for Database In-Memory. If you do not have an In-Memory licence, the original CC1 column cache is used. Another advantage of the CC2 format is that we can load row format blocks as well as HCC format blocks into pure columnar In-Memory format.  

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HCC IMCU inmemory memcompress oracle Roger MacNicol SmartScan

What’s new in 12.2 CELLMEMORY Part 3

Posted on May 4, 2017 by Roger MacNicol Posted in 12.2, inmemory, oracle, SmartScan 1 Comment

The Cellmemory Stats in RDBMS 

The RDBMS stats for Cellmemory are designed to closely follow the pattern used by the Inmemory stats 

Query Stats 

 Each column in each one MB of disk blocks will be rewritten into one IMC format Column CU in flash and a set of Column CUs comprise an overall Compression Unit so these stats reflect the number of 1 MB rewrites that were processed (not the number of column CUs).

  1. “cellmemory IM scan CUs processed for query”
    – #1 MB chuncks scanned in MEMCOMPRESS FOR QUERY format
  2. “cellmemory IM scan CUs processed for capacity”
    – #1 MB chuncks scanned in MEMCOMPRESS FOR CAPACITY format
  3. “cellmemory IM scan CUs processed no memcompress”
    – #1 MB chuncks scanned in NO CELLMEMORY format (12.1.0.2 format)

Load Stats

  1. “cellmemory IM load CUs for query”
    – #1 MB chunks successfully rewritten from 12.1.0.2 to MEMCOMPRESS FOR QUERY format  
  2. “cellmemory IM load CUs for capacity”
    – #1 MB chunks successfully rewritten from 12.1.0.2 to MEMCOMPRESS FOR CAPACITY format
  3. “cellmemory IM load CUs no memcompress”
    – #1 MB chunks successfully rewritten into 12.1.0.2 format

Before a rewrite happens a routine is called that looks through the blocks in the 1 MB chunk and determines if it is eligible for write. Reasons it may not be include transactional metadata from the commit cache, the presence of blocks formats that can’t be rewitten (although this list is getting smaller with each rpm), and the amount of space the rewrite will take up.

A rewrite into 12.1.0.2 format must fit in the original 1 MB of flash cache. An IMC format rewrite is not permitted to exceed 8 MB. This limit is highly unlikely to be reached by MEMCOMPRESS FOR CAPACITY but could be reached when trying to rewrite HCC blocks with much greater than 8X original compression capacity into MEMCOMPRESS FOR QUERY format. This is one reason that the default is FOR CAPACITY.

  1. “cellmemory IM scan CUs rejected for query”
    – #1 MB chunks that could not be rewritten into MEMCOMPRESS FOR QUERY for whatever reason
  2. “cellmemory IM scan CUs rejected for capacity
    – #1 MB chunks that could not be rewritten into MEMCOMPRESS FOR CAPACITY for whatever reason
  3. “cellmemory IM scan CUs rejected no memcompress”
    – #1 MB chunks that could not even be rewritten into 12.1.0.2 format for whatever reason
Cellmemory direct path reads IMCU inmemory memcompress oracle Roger MacNicol SmartScan
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