Hi, On 2019-02-19 13:45:28 -0500, Robert Haas wrote: > On Tue, Feb 19, 2019 at 1:29 PM Andres Freund <and...@anarazel.de> wrote: > > And I think it's not that likely that we'd not screw up a > > number of times implementing iSCSI ourselves - not to speak of the fact > > that that seems like an odd place to focus development on, given that > > it'd basically require all the infrastructure also needed for local DIO, > > which'd likely gain us much more. > > I don't really disagree with you here, but I also think it's important > to be honest about what size hammer is likely to be sufficient to fix > the problem. Project policy for many years has been essentially > "let's assume the kernel guys know what they are doing," but, I don't > know, color me a little skeptical at this point.
Yea, and I think around e.g. using the kernel page cache / not using DIO, several people, including kernel developers and say me, told us that's stupid. > We've certainly made lots of mistakes all of our own, and it's > certainly true that reimplementing large parts of what the kernel does > in user space is not very appealing ... but on the other hand it looks > like filesystem error reporting isn't even really reliable for local > operation (unless we do an incredibly complicated fd-passing thing > that has deadlock problems we don't know how to solve and likely > performance problems too, or convert the whole backend to use threads) > or for NFS operation (though maybe your suggestion will fix that) so > the idea that iSCSI is just going to be all right seems a bit > questionable to me. Go ahead, call me a pessimist... My point is that for iSCSC to be performant we'd need *all* the infrastructure we also need for direct IO *and* a *lot* more. And that it seems insane to invest very substantial resources into developing our own iSCSI client when we don't even have DIO support. And DIO support would allow us to address the error reporting issues, while also drastically improving performance in a lot of situations. And we'd not have to essentially develop our own filesystem etc. Greetings, Andres Freund
