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From Oystein.Grov...@Sun.COM (Øystein Grøvlen)
Subject Re: page replacement [jira] Commented: (DERBY-239) Need a online backup feature that does not block update operations when online backup is in progress.
Date Mon, 25 Jul 2005 15:06:09 GMT
>>>>> "DJD" == Daniel John Debrunner <djd@debrunners.com> writes:

    DJD> I think modifications to the cache would be useful for b), so
    DJD> that entries in the cache (through generic apis, not specific
    DJD> to store) could mark how "useful/valuable" they are. Just a
    DJD> simple scheme, lower numbers less valuable, higher numbers
    DJD> more valuable, and if it makes it easier to fix a range,
    DJD> e.g. 0-100, then that would be ok. Then the store could added
    DJD> pages to the cache with this weighting, e.g. (to get the
    DJD> general idea)

    DJD>      pages for backup - weight 0
    DJD>      overflow column pages - weight 10
    DJD>      regular pages - weight 20
    DJD>      leaf index pages - weight 30
    DJD>       root index pages 80

    DJD> This weight would then be factored into the decision to throw pages out
    DJD> or not.

I agree that we need some mechanism to prevent operations from filling
the cache with pages that is not likely to be accesssed again in the
near future.  However, I am afraid that a very detailed "cost-based"
scheme may create a significant overhead compared to a simple LRU
scheme.

One may operate with separate LRU queues for different weights, but I
guess the number of possible weights will have to be restricted in
that case.

I am also not convinced that it is the type of page that is the most
important criteria for caching.  What matters is access frequency.
The page type may give a hint, but leaf pages of one index may be more
frequently accessed than root pages of other indexes.

The type of access is also a relevant criteria.  Pages accessed
sequentially is often less likely to be accessed again in the near
future than pages accessed by direct lookup.  A separate LRU queue for
sequentially accessed pages may prevent backup and other sequentially
scans (e.g., select * from t) from throwing out directly accessed
pages (e.g., index pages and data pages accessed through indexes.)

    DJD> This project could be independent of the online backup and could have
    DJD> benfits elsewhere.

I agree.


-- 
Øystein


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