On Fri, Dec 3, 2021 at 8:28 PM Dilip Kumar <dilipbal...@gmail.com> wrote:
> On Fri, Dec 3, 2021 at 7:38 PM Ashutosh Sharma <ashu.coe...@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.
