Re: Drop type "smgr"?
От | Thomas Munro |
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Тема | Re: Drop type "smgr"? |
Дата | |
Msg-id | CA+hUKGJekqzkHKritUox5_BEaN9e2yV_MiuCZzRiVDQ+9kLTMA@mail.gmail.com обсуждение исходный текст |
Ответ на | Re: Drop type "smgr"? (Shawn Debnath <sdn@amazon.com>) |
Ответы |
Re: Drop type "smgr"?
(Konstantin Knizhnik <k.knizhnik@postgrespro.ru>)
|
Список | pgsql-hackers |
On Fri, Mar 1, 2019 at 10:41 AM Shawn Debnath <sdn@amazon.com> wrote: > On Fri, Mar 01, 2019 at 10:33:06AM +1300, Thomas Munro wrote: > > It doesn't make any sense to put things like clog or any other SLRU in > > a non-default tablespace though. It's perfectly OK if not all smgr > > implementations know how to deal with tablespaces, and the SLRU > > support should just not support that. > > If the generic storage manager, or whatever we end up calling it, ends > up being generic enough - its possible that tablespace value would have > to be respected. Right, you and I have discussed this a bit off-list, but for the benefit of others, I think what you're getting at with "generic storage manager" here is something like this: on the one hand, our proposed revival of SMGR as a configuration point is about is supporting alternative file layouts for bufmgr data, but at the same time there is some background noise about direct IO, block encryption, ... and who knows what alternative block storage someone might come up with ... at the block level. So although it sounds a bit contradictory to be saying "let's make all these different SMGRs!" at the same time as saying "but we'll eventually need a single generic SMGR that is smart enough to be parameterised for all of these layouts!", I see why you say it. In fact, the prime motivation for putting SLRUs into shared buffers is to get better buffering, because (anecdotally) slru.c's mini-buffer scheme performs abysmally without the benefit of an OS page cache. If we add optional direct IO support (something I really want), we need it to apply to SLRUs, undo and relations, ideally without duplicating code, so we'd probably want to chop things up differently. At some point I think we'll need to separate the questions "how to map blocks to filenames and offsets" and "how to actually perform IO". I think the first question would be controlled by the SMGR IDs as discussed, but the second question probably needs to be controlled by GUCs that control all IO, and/or special per relation settings (supposing you can encrypt just one table, as a random example I know nothing about); but that seems way out of scope for the present projects. IMHO the best path from here is to leave md.c totally untouched for now as the SMGR for plain old relations, while we work on getting these new kinds of bufmgr data into the tree as a first step, and a later hypothetical direct IO or whatever project can pay for the refactoring to separate IO from layout. -- Thomas Munro https://enterprisedb.com
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