Re: Enable data checksums by default

On Fri, Aug 1, 2025 at 6:37 AM Ants Aasma <ants(dot)aasma(at)cybertec(dot)at> wrote:

> Even if we made the checksum algorithm itself faster, the main issue
> is actually memory bandwidth. Intel server CPUs have about half the
> bandwidth of AMD ones. A checksum has to pull in the whole page in a
> few hundred cycles. Without checksums only a part of the page might be
> accessed and the accesses are spread over a longer time, making them
> easier to hide by out-of-order execution.
>
> But all the above still ends up at being a few hundred nanoseconds per
> buffer read. Basically this ends up only mattering measurably for
> in-RAM but out of shared buffers workloads. And the easy workaround is
> to increase shared buffers. As you said, the main issue is the other
> overheads that checksums pull in.
>

I want to point out that at some point in time there might well be demand
for checksumming pages living in shared_buffers. Modern storage systems
assume that the durable media is going to have errors and already have
robust ways to detect that. But they also assume that ECC memory is
bulletproof (it's not), and that's the biggest benefit to Postgres
checksums: they protect data in the filesystem cache[1]. You obviously lose
that if you size shared_buffers to consume most of available memory.

Obviously trying to address that is way beyond the scope of what's being
discussed here. I'm honestly unsure of how relevant it is, but I wanted to
make sure folks were aware of it.

1: I can't go into details, but I have seen a case where Postgres checksums
led to an investigation that ultimately revealed a memory-related issue. In
other words, data was actually getting corrupted while in the filesystem
cache. Obviously data could (and likely was) also get corrupted in shared
buffers, but the corruption in the FS cache was what prompted the
investigation that ultimately found the hardware issue. Fortunately
shared_buffers was small enough to make it more likely that corruption
would happen outside of Postgres, so it could be detected.