Yes, you are right. It does not really need to be written, but
in the current implementation it doesn't cost very much to
write it since we sync the entire file at the end. I tried to
add some more descriptive comments about this in:
BaseDataFileFactory.java!checkpoint
The actual loop is in the cachemanager cleanAll implementation.
Raymond Raymond wrote:
I have a question needs to be confirmed. Anyone can help me?
In checkpoint, derby checks from the last cached item in the
holder array to the first one(ref:
org.apache.derby.impl.services.cache.Clock.java)
to find dirty pages and write them out.
A cache page, which is clean before checkpoint starts and is updated
after checkpoint starts, will have a chance to be written out by the
current checkpoint process. Am I right?
For instance, let's assume derby keeps 1000 cache pages. When
checkpoint starts, it checks from the 1000th cache page to the
first one. When it goes to check the 800th cache page, the 700th
cache page is updated. The 700th cache page will be checked later
and written out as a dirty page( assume it is valid). If the 900th cache
page is updated when the 800th cache page is checked. The 900th
cache page will not be written out by the current checkpoint process.
Am I right?
Thanks.
Raymond
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