Re: storing an explicit nonce

On Tue, May 25, 2021 at 2:45 PM Bruce Momjian  wrote:
> Well, if we create a separate nonce counter, we still need to make sure
> it doesn't go backwards during a crash, so we have to WAL log it

I think we don't really need a global counter, do we? We could simply
increment the nonce every time we write the page. If we want to avoid
using the same IV for different pages, then 8 bytes of the nonce could
store a value that's different for every page, and the other 8 bytes
could store a counter. Presumably we won't manage to write the same
page more than 2^64 times, since LSNs are limited to be <2^64, and
those are consumed more than 1 byte at a time for every change to any
page anywhere.

> The buffer encryption overhead is 2-4%, and WAL encryption is going to
> add to that, so I thought hint bit logging overhead would be minimal
> in comparison.

I think it depends. If buffer evictions are rare, then it won't matter
much. But if they are common, then using the LSN as the nonce will add
a lot of overhead.

> Have you looked at the code, specifically EncryptPage():
>
>         https://github.com/postgres/postgres/compare/bmomjian:cfe-11-gist..bmomjian:_cfe-12-rel.patch
>
> +       if (!relation_is_permanent && !is_gist_page_or_similar)
> +               PageSetLSN(page, LSNForEncryption(relation_is_permanent));
>
>
> It assigns an LSN to unlogged pages.  As far as the buffer manager
> seeing fake LSNs that already happens for GiST indexes, so I just built
> on that --- seemed to work fine.

I had not, but I don't see why this issue is specific to GiST rather
than common to every kind of unlogged and temporary relation.

> I have to ask why we should consider adding it to the special space,
> since my current version seems fine, and has minimal code impact, and
> has some advantages over using the special space.  Is it because of the
> WAL hint overhead, or for a cleaner API, or something else?

My concern is about the overhead, and also the code complexity. I
think that making sure that the LSN gets changed in all cases may be
fairly tricky.

-- 
Robert Haas
EDB: http://www.enterprisedb.com