Re: patch to allow disable of WAL recycling

On 3/6/19 11:30 AM, Robert Haas wrote:
> On Wed, Mar 6, 2019 at 10:55 AM Andrew Dunstan
> <andrew(dot)dunstan(at)2ndquadrant(dot)com> wrote:
>>> I *really* dislike this. For one thing, it means that users don't
>>> have control over the behaviors individually. For another, the
>>> documentation is now quite imprecise about what the option actually
>>> does, while expecting users to figure out whether those behaviors are
>>> acceptable or preferable in their environment. It lists recycling of
>>> WAL files and zero-filling of those files as examples of behavior
>>> changes, but it does not say that those are the only changes, or even
>>> that they are made in all cases.
>> So you want two options, like wal_recycle_files and wal_zero_fill, both
>> defaulting to true? Is there a reasonably use case for turning one off
>> without the other?
> I don't know whether there's a use case for that, and that's one of
> the things that worries me. I know, though, that if we have two
> parameters, then if there is a use case for it, people will be able to
> meet that use case without submitting a patch. On the other hand, if
> we had convincing evidence that those two things should always go
> together, that would be fine, too. But I don't see that anyone has
> made an argument that such a thing is necessarily true outside of ZFS.
>
> I actually wouldn't find it very surprising if disabling WAL recycling
> is sometimes beneficial even on ext4. The fact that we haven't found
> such cases on this thread doesn't mean they don't exist. On the other
> hand I think the wal_zero_fill behavior is not about performance but
> about reliability, so you can't afford to turn that on just because
> non-recycling happens to be faster on your machine.
>
>

Well, let's put the question another way. Is there any reason to allow
skipping zero filling if we are recycling? That seems possibly
dangerous. I can imagine turning off recycling but leaving on
zero-filling, although I don't have a concrete use case for it ATM.

cheers

andrew

--
Andrew Dunstan https://www.2ndQuadrant.com
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