Re: increasing the default WAL segment size

Hi,

On 2016-12-19 15:14:50 +0530, Beena Emerson wrote:
> The attached patch removes --with-wal-segsize configure option and adds a
> new initdb option --wal-segsize. The module initdb passes the wal-segsize
> value into an environment variable which is used to overwrite the guc value
> wal_ segment_size and set the internal variables : XLogSegSize and
> XLOG_SEG_SIZE (xlog_internal.h). The default wal_segment_size is not
> changed but I have increased the maximum size to 1GB.
>
> Since XLOG_SEG_SIZE is now variable, it could not be used directly in
> src/bin modules and few macros and few changes had to be made:

I do think this has the potential for negative performance
implications. Consider code like
/* skip over the page header */
if (CurrPos % XLogSegSize == 0)
{
CurrPos += SizeOfXLogLongPHD;
currpos += SizeOfXLogLongPHD;
}
else
right now that's doable in an efficient manner, because XLogSegSize is
constant. If it's a variable and the compiler doesn't even know it's a
power-of-two, it'll have to do a full "div" - and that's quite easily
noticeable in a lot of cases.

Now it could entirely be that the costs of this will be swamped by
everything else, but I'd not want to rely on it.

I think we need tests with concurrent large-file copies. And then also
look at the profile to see whether the relevant places become new
hotspots (not that we introduce something that's just hidden for now).

We might be able to do a bit better, efficency wise, by storing
XLogSegSize as a "shift factor". I.e. the 16M setting would be 24
(i.e. XLogSegSize would be defined as 1 << 24).

Greetings,

Andres Freund