On Thu, Jan 2, 2020 at 9:09 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > Hi, > > I am starting a new thread for some of the decisions for a parallel vacuum in > the hope to get feedback from more people. There are mainly two points for > which we need some feedback. > > 1. Tomas Vondra has pointed out on the main thread [1] that by default the > parallel vacuum should be enabled similar to what we do for Create Index. As > proposed, the patch enables it only when the user specifies it (ex. Vacuum > (Parallel 2) <tbl_name>;). One of the arguments in favor of enabling it by > default as mentioned by Tomas is "It's pretty much the same thing we did with > vacuum throttling - it's disabled for explicit vacuum by default, but you can > enable it. If you're worried about VACUUM causing issues, you should set cost > delay.". Some of the arguments against enabling it are that it will lead to > use of more resources (like CPU, I/O) which users might or might like. >
I'm a bit wary of making parallel vacuum enabled by default. Single process vacuum does sequential reads/writes on most of indexes but parallel vacuum does random access random reads/writes. I've tested parallel vacuum on HDD and confirmed the performance is good but I'm concerned that it might be cause of more disk I/O than user expected. > Now, if we want to enable it by default, we need a way to disable it as well > and along with that, we need a way for users to specify a parallel degree. I > have mentioned a few reasons why we need a parallel degree for this operation > in the email [2] on the main thread. > > If parallel vacuum is *not* enabled by default, then I think the current way > to enable is fine which is as follows: > Vacuum (Parallel 2) <tbl_name>; > > Here, if the user doesn't specify parallel_degree, then we internally decide > based on number of indexes that support a parallel vacuum with a maximum of > max_parallel_maintenance_workers. > > If the parallel vacuum is enabled by default, then I could think of the > following ways: > (a) Vacuum (disable_parallel) <tbl_name>; Vacuum (Parallel > <parallel_degree>) <tbl_name>; > (b) Vacuum (Parallel <parallel_degree>) <tbl_name>; If user specifies > parallel_degree as 0, then disable parallelism. > (c) ... Any better ideas? > If parallel vacuum is enabled by default, I would prefer (b) but I don't think it's a good idea to accept 0 as parallel degree. If we want to disable parallel vacuum we should max_parallel_maintenance_workers to 0 instead. Regards, -- Masahiko Sawada http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
