Thank you Dr. Hipp. The DB info makes perfect sense. Since I have to
accommodate the initial read in the DB file you don't need to check on
the change counter. I'll handle it already.
With regard to the journal files, I understand that they will for the
most part be written sequentially (with the exception of the header).
That is a useful hint for these types of files. It sounds like journal
reads and writes won't be sector sized and/or sector aligned. I can
accommodate that with little effort as well.
[EMAIL PROTECTED] wrote:
Mark Spiegel <[EMAIL PROTECTED]> wrote:
I take this to mean that paged-aligned sector I/O will always be used
for all the DB types. Will there ever be an I/O to a DB file that will
not follow these rules.
When first opening a database, 100 bytes are ready from the very
beginning of the database file. Part of this header tells us how
big the pages are. The initial read is page-aligned but it is not
a read of a complete page, obviously, since at that time we do not
know how big the page is.
Otherwise, I think all database I/O is of complete pages. There
might be an exception for updating the change counter in the header.
I'd have to check.
What about the journal types? Will their I/O
be sector sized and aligned I/O as well?
No. Journals are mostly sequentially written, increasing the
file size as it is written. There is a sector-sized header
at the beginning of the journal that is skipped at first
and then overwritten at the end. If the cache spills,
this sector-sized header might also be repeated at various
points in the middle of the file.
--
D. Richard Hipp <[EMAIL PROTECTED]>
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