Re: Changing WAL Header to reduce contention during ReserveXLogInsertLocation()

I wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Sat, Dec 30, 2017 at 5:32 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>>> For this to work, we must ensure that WAL files are either recycled in
>>> such a way that the "xl_walid" of the previous (to be recycled) WAL
>>> differs from the new WAL or we zero-out the new WAL file. Seems quite
>>> easy to do with the existing infrastructure.

> I have no faith that we can prevent old WAL data from reappearing in the
> file system across an OS crash, so I find Simon's assertion that we can
> dodge the problem through file manipulation to be simply unbelievable.

Forgot to say: if we *could* do it reliably, it would likely add
significant I/O traffic, eg an extra zeroing pass for every WAL segment.
That's a pretty heavy performance price to set against whatever win
we might get from contention reduction.

regards, tom lane