Re: Postgres IO sweet spot

Good day all

Just following up of there is any advice from the community. My original
post was very long, but just wanted to paint the picture.
In summary I just want to find out if anybody has some concrete advice
on storage devices that is acceptable for usage with Postgres,
especially latency. I've highlighted some tests I've done, but I need to
interpret the numbers correctly.
Any feedback on the following will help

* Acceptable write IO latency
o WAL
o Data
o Temp
o ...
* Acceptable read IO latency
* Any other storage/drive related advice

Regards
Riaan

On 2026/02/11 01:13, Riaan Stander wrote:
> Good day
>
> We host our own Postgres (v17) server on-prem as the backbone of our
> SaS application. It's a fairly busy OLTP application with a database
> per tenant strategy. This obviously does complicate our setup.
> Our hosting platform is as follows:
> 3 x Host Servers running Microsoft Storage Spaces in a 3 way mirror
> Ubuntu VM hosting Postgres
>
> A few months ago we had some severe performance issues with lots of
> queries and writing operations just pending. After some deep
> investigation we started realizing that it was disk IO causing the
> issue. We used iostat and could see the write await was above 30ms and
> sometimes even spiking much higher. This was resolved by moving our
> backups (made with Veeam) from backing up the primary to a slave on
> other infrastructure. Our current happy state where clients are not
> experiencing issues is a iostat write await of 5ms and lower.
>
> All was good for a few months until recently when this issue started
> again. This time it could not be the backups. We had various hardware
> vendors involved, but at some point it came to light that the Storage
> Spaces hardware are all mechanical disks with NVME only used for
> Storage Spaces journaling and caching. There are now some discussions
> of upgrading drives to SSD, but my concern is that this is not
> guaranteed to solve the issue. Especially with the 3 way mirror it
> seems all writes will go to the other hosts before returning. So
> latency is almost impossible to remove.
>
> So now my question. I started running some IO tests using fio,
> pg_test_fsync & pg_test_timing. Before we spend days/months trying to
> tune Postgres settings I'm trying to get some definitive published
> information about what IO numbers I should expect when running plain
> hardware tests with Postgres completely out of the loop. I've seen
> some info about 1ms and less write latency is what you want for WAL.
> My logic says that if you have a stiffie drive for storage you can
> tune it, but you still have a stiffie drive.
>
> These are the tests I've run so far
> 1. WAL-Style Latency Test (4K random sync writes)
> fio --name=wal-latency --filename=$TESTDIR/fio_wal_test --size=2G
> --rw=randwrite --bs=4k --iodepth=1 --ioengine=libaio --direct=1
> --fsync=1 --runtime=60 --group_reporting
>
> 2. Random Read IOPS Test (index lookup simulation)
> fio --name=index-read --filename=$TESTDIR/fio_index_test --size=8G
> --rw=randread --bs=4k --iodepth=32 --ioengine=libaio --direct=1
> --runtime=60 --group_reporting
>
> 3. Mixed OLTP Test (70% read / 30% write)
> fio --name=oltp-mixed --filename=$TESTDIR/fio_oltp_mixed --size=8G
> --rw=randrw --rwmixread=70 --bs=8k --iodepth=32 --ioengine=libaio
> --direct=1 --runtime=60 --group_reporting
>
> 4. Checkpoint Burst Test (sequential write pressure)
> fio --name=checkpoint-burst --filename=$TESTDIR/fio_checkpoint
> --size=20G --rw=write --bs=1M --iodepth=64 --ioengine=libaio
> --direct=1 --runtime=60 --group_reporting
>
> 5. PostgreSQL fsync Code Path Test
> pg_test_fsync -f $TESTDIR/pg_test_fsync
>
> 6. Timer / Scheduling Jitter Test
> pg_test_timing -d 3
>
> Regards
> Riaan
>
>
>