From: Mike Matrigali <[EMAIL PROTECTED]>
Reply-To: "Derby Development" <[email protected]>
To: Derby Development <[email protected]>
Subject: Re: Can anyone give me some suggestions?
Date: Tue, 23 Aug 2005 10:03:35 -0700
Yes, the key is that in normal operation the call to these routines
is always done by a background process which is a different thread
than normal client threads. So database client threads procede
while the checkpoint procedes. The buffer cache is not locked for
the duration of the checkpoint, only individual pages for the time
it takes to copy the page from the buffer cache into the OS cache.
Thanks lots, I got what you said. But I still have some questions about
that.
1. If a buffer page, we call it B1 here, is updated before a checkpoint is
taken
and the log of the update is generated as L1. During the following
checkpoint,
depending on the WAL policy, log buffer will be first forced to disk, so L1
will
be flushed to disk. Then the checkpoint process will try to write the cache
buffer
to disk. My question is, since the checkpoint thread and the database client
threads
are asynchronous, it is possible another update on B1 happens before B1 is
latched and
written out, later B1 will be written out but the log of the second update
is not
written out yet. How does derby ensure the WAL policy in this case?
2. During a checkpoint, now derby will search all the cache buffer for dirty
pages
and write them out. Why don't we keep a dirty page list? Just IDs that can
identify
the corresponding dirty page are stored in the list, so it will not take a
lot of
space. The first time a buffer page is updated, its ID will be appended to
the list,
and after the dirty page has been written out, it will be released from the
list.
During a checkpoint, we just search from the head of the list to the end and
write
the corresponding dirty pages out.
Raymond
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