Re: [Proposal] Fully WAL logged CREATE DATABASE - No Checkpoints

On Fri, Dec 3, 2021 at 8:28 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:

> On Fri, Dec 3, 2021 at 7:38 PM Ashutosh Sharma <ashu.coek88@gmail.com>
> wrote:
> >
> > I see that this patch is reducing the database creation time by almost
> 3-4 times provided that the template database has some user data in it.
> However, there are couple of points to be noted:
>
> Thanks a lot for looking into the patches.
> >
> > 1) It makes the crash recovery a bit slower than before if the crash has
> occurred after the execution of a create database statement. Moreover, if
> the template database size is big, it might even generate a lot of WAL
> files which the user needs to be aware of.
>
> Yes it will but actually that is the only correct way to do it, in
> current we are just logging the WAL as copying the source directory to
> destination directory without really noting down exactly what we
> wanted to copy, so we are force to do the checkpoint right after
> create database because in crash recovery we can not actually replay
> that WAL.  Because WAL just say copy the source to destination so it
> is very much possible that at the DO time source directory had some
> different content than the REDO time so this would have created the
> inconsistencies in the crash recovery so to avoid this bug they force
> the checkpoint so now also if you do force checkpoint then again crash
> recovery will be equally fast.  So I would not say that we have made
> crash recovery slow but we have removed some bugs and with that now we
> don't need to force the checkpoint.  Also note that in current code
> even with force checkpoint the bug is not completely avoided in all
> the cases, check below comments from the code[1].
>
> > 2) This will put a lot of load on the first checkpoint that will occur
> after creating the database statement. I will experiment around this to see
> if this has any side effects.
>
> But now a checkpoint can happen at its own need and there is no need
> to force a checkpoint like it was before patch.
>
> So the major goal of this patch is 1) Correctly WAL log the create
> database which is hack in the current system,  2) Avoid force
> checkpoints, 3) We copy page by page so it will support TDE because if
> the source and destination database has different encryption then we
> can reencrypt the page before copying to destination database, which
> is not possible in current system as we are copying directory  4) Now
> the new database pages will get the latest LSN which is the correct
> things earlier new database pages were getting copied directly with
> old LSN only.
>

OK. Understood, thanks.!

--
With Regards,
Ashutosh Sharma.