> On Aug 25, 2015, at 10:45 AM, Bill Moran <wmo...@potentialtech.com> wrote: > > On Tue, 25 Aug 2015 10:08:48 -0700 > David Kerr <d...@mr-paradox.net> wrote: > >> Howdy All, >> >> For a very long time I've held the belief that splitting PGDATA and xlog on >> linux systems fairly universally gives a decent performance benefit for many >> common workloads. >> (i've seen up to 20% personally). >> >> I was under the impression that this had to do with regular fsync()'s from >> the WAL >> interfearing with and over-reaching writing out the filesystem buffers. >> >> Basically, I think i was conflating fsync() with sync(). >> >> So if it's not that, then that just leaves bandwith (ignoring all of the >> other best practice reasons for reliablity, etc.). So, in theory if you're >> not swamping your disk I/O then you won't really benefit from relocating >> your XLOGs. > > Disk performance can be a bit more complicated than just "swamping." Even if
Funny, on revision of my question, I left out basically that exact line for simplicity sake. =) > you're not maxing out the IO bandwidth, you could be getting enough that some > writes are waiting on other writes before they can be processed. Consider the > fact that old-style ethernet was only able to hit ~80% of its theoretical > capacity in the real world, because the chance of collisions increased with > the amount of data, and each collision slowed down the overall transfer speed. > Contrasted with modern ethernet that doesn't do collisions, you can get much > closer to 100% of the rated bandwith because the communications are > effectively > partitioned from each other. > > In the worst case scenerion, if two processes (due to horrible luck) _always_ > try to write at the same time, the overall responsiveness will be lousy, even > if the bandwidth usage is only a small percent of the available. Of course, > that worst case doesn't happen in actual practice, but as the usage goes up, > the chance of hitting that interference increases, and the effective response > goes down, even when there's bandwidth still available. > > Separate the competing processes, and the chance of conflict is 0. So your > responsiveness is pretty much at best-case all the time. Understood. Now in my previous delve into this issue, I showed minimal/no disk queuing, the SAN showed nothing on it's queues and no retries. (of course #NeverTrustTheSANGuy) but I still yielded a 20% performance increase by splitting the WAL and $PGDATA But that's besides the point and my data on that environment is long gone. I'm content to leave this at "I/O is complicated" I just wanted to make sure that i wasn't correct but for a slightly wrong reason. Thanks! -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
