On Wed, Oct 6, 2021 at 11:17:59AM -0400, Robert Haas wrote:
> If you enable checksums or set wal_log_hints=on, then you might incur
> a some write-ahead log records that would otherwise be avoided, and
> those records will include full page images. This can happen once per
> page per checkpoint cycle. However, if the first modification to a
> particular page within a given checkpoint cycle is a regular
> WAL-logged operation rather than a hint bit change, then the extra WAL
> record and full-page image are not needed so the overhead is zero.
> Also, if the first modification is a hint bit change, and then the
> page is evicted, prompting a full page write, but a regular WAL-logged
> operation occurs later within the same checkpoint, the later operation
> no longer needs a full page write. So you still paid the cost of an
> extra WAL record, but you didn't pay the cost of an extra full page
> write. In other words, enabling checksums or turning wal_log_hints=on
> has a relatively low cost except when you have pages that incur only
> hint-type changes, and no regular changes, within the course of a
> single checkpoint cycle.
>
> On the other hand, in order to avoid IV reuse, your patch needed to
> bump the page LSN for every change, or at least for every eviction.
> That means you could potentially incur the overhead of an extra full
> page write multiple times per checkpoint cycle, and even if there were
> non-hint changes to that page in the same checkpoint cycle. Now you
> could say, well, let's not bump the page LSN for every hint-type
> change, and then your patch would have lower overhead than an approach
> based on XTS, but I think that also loses a ton of security, because
> now you're reusing IVs with an encryption system that is documented
> not to tolerate the reuse of IVs.
>
> I'm not here to try to pretend that encryption is going to be cheap. I
> just don't believe this particular argument about why AES-XTS should
> be more expensive.
OK, good to know.
--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EDB https://enterprisedb.com
If only the physical world exists, free will is an illusion.
