Igor Tandetnik <[EMAIL PROTECTED]> wrote: Karl Tomlinson
wrote:
> Thank you for your comments.
>
>>> Karl Tomlinson wrote:
>>>
>>>> One thing I'm trying to understand is whether there was a reason
>>>> for SQLite choosing to use a rollback journal (of the steps to
>>>> undo a transaction) rather than a replay journal (of the steps to
>>>> perform a transaction).
>
> I didn't make this quite clear. I really meant, as a first
> possibility, that writing the pages to the database itself would
> be performed during the commit process after syncing the replay
> journal (but the database need not be synced until the journal is
> about to be removed).
So if I insert large amounts of data, this data would be written to disk
twice: once into a replay journal, and then again into the database file
on commit. With rollback journal, it is written once into the database
(with rollback journal merely noting which new pages have been created),
and commit is nearly instantaneous. I'd rather optimize for the common
case (transactions successfully committed) than the exceptional one.
Ok, I'll argue why write the entire page, why not just write what changed on
the page? Allowing more information to be written to a redo journal (ie more
than one modification) per redo page ???? How often does a write actually
modify the entire page?
Definately agree about optimizing for the common case of successfully
commited transactions.
>> SQLite would have to keep track of where in the replay journal each
>> new page was written, so that if it needed to reload that page for
>> any reason, it could. As long as the transaction (and thus the
>> replay journal) is not too big, this is not a problem. A hash table
>> in memory would suffice. But sometimes transactions do get very
>> large. For example, with the VACUUM command. And in those cases,
>> the amount of memory needed to record the mapping from pages to
>> offsets into the replay journal could become too large to fit into
>> memory, especially in memory-constrained devices such as cellphones.
>
> The size of the transaction would be something to consider in the
> decision on when to sync the journal and write its pages to the
> database.
So presumably, as the transaction grows, at some point you may decide to
dump the replay journal into the database file even while the
transaction is still open. What if the user decides to roll it back soon
afterwards? Wouldn't you need both a replay and a rollback journals
then?
Yes you would, but then again you'd also want to put the changes for the undo
journal into the redo journal so that if the "DB" crashed it would be able to
recover not just the data files but the undo as well. Which if doing this
will naturaly lead you to multiple transactions, concurrency and multiple redo
journals and a WHOLE lot of other features.
Which seems to me to be a pretty large scope creep.
The
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