On 3/6/19 11:30 AM, Robert Haas wrote: > On Wed, Mar 6, 2019 at 10:55 AM Andrew Dunstan > <andrew.duns...@2ndquadrant.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 PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
