Re: pg_upgrade parallelism

On Wed, 2021-11-17 at 14:34 -0600, Justin Pryzby wrote:
> On Wed, Nov 17, 2021 at 02:44:52PM -0500, Jaime Casanova wrote:
> >
> > - why we read()/write() at all? is not a faster way of copying the file?
> > i'm asking that because i don't actually know.
>
> No portable way. Linux has this:
> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fman7.org%2Flinux%2Fman-pages%2Fman2%2Fcopy_file_range.2.html&data=04%7C01%7Cpchampion%40vmware.com%7C35fb5d59bd2745636fd408d9aa09a245%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637727780625465398%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=PS6OCE55n12KBOjh5qZ2uGzDR9U687nzNIV5AM9Zke4%3D&reserved=0
>
> But I just read:
>
> > First support for cross-filesystem copies was introduced in Linux
> > 5.3. Older kernels will return -EXDEV when cross-filesystem
> > copies are attempted.
>
> To me that sounds like it may not be worth it, at least not quite yet.
> But it would be good to test.

I think a downside of copy_file_range() is that filesystems might
perform a reflink under us, and to me that seems like something that
needs to be opted into via clone mode.

(https://lwn.net/Articles/846403/ is also good reading on some sharp
edges, though I doubt many of them apply to our use case.)

--Jacob