3 ms·

I'll see if I have a chance to run such a test on AWS in coming days (and would need to keep running it for much longer than just 5 seconds shown in the blog).

I'll see if I have a chance to run such a test on AWS in coming days (and would need to keep running it for much longer than just 5 seconds shown in the blog). If you care about WAL write/commit latency, you could provision a small-ish EBS io2 Block Express device (with provisioned IOPS) just for your WAL files and the rest of your data can still reside on cheaper EBS storage. And you might not even need to hugely overprovision your WAL device IOPS (as databases can batch commit writes for multiple transactions). But the main point is that once your WAL files are on a completely separate blockdevice from all the other datafile I/O, they won't suffer from various read & write IO bursts that can happen during regular database activity. On Oracle databases, I put controlfiles to these separate devices too, as they are on the critical path during redo log switches...

CodesInChaos 1y ago
How do you backup those split volumes? I like EBS volume snapshots, since they're atomic, incremental, fast to restore and make it easy to spin up a clone. But obviously that approach won't work for split volumes.
- tanelpoder 1y ago
  Yep indeed, that's a tradeoff, if your DB fits into a single volume. I'm not that deeply familiar with databases other than Oracle (that has its own ways to work work around this), so for ease of use, everything on a single volume keeps things simpler. One thing that I try to achieve anyway, is to spread & "smoothen" the database checkpoint & fsync activity over time via database checkpointing parameters, so you won't have huge "IO storms" every 15 minutes, but just steady writing of dirty buffers going on all the time. So, even if all your files are stored on the same blockdevice, you'll less likely see a case where your WAL writes wait behind 50000 checkpoint write requests issued just before.
  - CoolCold 1y ago
    well, likely you can just use those as ZFS with ZIL or in more tradional setup with LVM + LVW writeback cache - which from my experience greatly improves latency

CodesInChaos 1y ago

How do you backup those split volumes? I like EBS volume snapshots, since they're atomic, incremental, fast to restore and make it easy to spin up a clone. But obviously that approach won't work for split volumes.

tanelpoder 1y ago
Yep indeed, that's a tradeoff, if your DB fits into a single volume. I'm not that deeply familiar with databases other than Oracle (that has its own ways to work work around this), so for ease of use, everything on a single volume keeps things simpler. One thing that I try to achieve anyway, is to spread & "smoothen" the database checkpoint & fsync activity over time via database checkpointing parameters, so you won't have huge "IO storms" every 15 minutes, but just steady writing of dirty buffers going on all the time. So, even if all your files are stored on the same blockdevice, you'll less likely see a case where your WAL writes wait behind 50000 checkpoint write requests issued just before.
- CoolCold 1y ago
  well, likely you can just use those as ZFS with ZIL or in more tradional setup with LVM + LVW writeback cache - which from my experience greatly improves latency