Re: Allow io_combine_limit up to 1MB

Here's a new version that also adjusts the code that just landed in da722699:

- /*
- * Each IO handle can have an PG_IOV_MAX long iovec.
- *
- * XXX: Right now the amount of space available for each IO is
PG_IOV_MAX.
- * While it's tempting to use the io_combine_limit GUC, that's
- * PGC_USERSET, so we can't allocate shared memory based on that.
- */
+ /* each IO handle can have up to io_max_combine_limit iovec objects */
return mul_size(sizeof(struct iovec),
- mul_size(mul_size(PG_IOV_MAX,
AioProcs()),
+
mul_size(mul_size(io_max_combine_limit, AioProcs()),
io_max_concurrency));

While doing that, this juxtaposition jumped out at me:

+ uint32 per_backend_iovecs = io_max_concurrency *
max_io_combine_limit;

Surely one of these names has to give! First commit wins, so the new
name in this version is "io_max_combine_limit".

I was contemplating making the same change to the MAX_IO_COMBINE_LIMIT
macro when it occurred to me that we could just delete it. It has few
references and they could just use PG_IOV_MAX instead; it's perhaps a
little less clear as names go, but it's also pretty confusing that
there's a macro with the same name as a GUC...