Re: increasing the default WAL segment size

On Thu, Aug 25, 2016 at 12:21 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> On 25 August 2016 at 02:31, Robert Haas <robertmhaas@gmail.com> wrote:
>
>> Furthermore, there is an enforced, synchronous fsync at the end of
>> every segment, which actually does hurt performance on write-heavy
>> workloads.[2] Of course, if that were the only reason to consider
>> increasing the segment size, it would probably make more sense to just
>> try to push that extra fsync into the background, but that's not
>> really the case.  From what I hear, the gigantic number of files is a
>> bigger pain point.
>
> I think we should fully describe the problem before finding a solution.
>
> This is too big a change to just tweak a value without discussing the
> actual issue.
>
> And if the problem is as described, how can a change of x4 be enough
> to make it worth the pain of change? I think you're already admitting
> it can't be worth it by discussing initdb configuration.
>
> If we do have the pain of change, should we also consider making WAL
> files variable length? What do we gain by having the files all the
> same size? ISTM better to have WAL files that vary in length up to 1GB
> in size.
>
> (This is all about XLOG_SEG_SIZE; I presume XLOG_BLCKSZ can stay as it
> is, right?)

Avoiding variable sizes does avoid some failure modes on the
filesystem side in the face of crashes/power loss.

So making them variable size, while possible, wouldn't be simple at
all (it would involve figuring out the way filesystems behave when
facing crash when a file is changing in size).