Thread

1. Postgres IO sweet spot

   rstander@exa.co.za — 2026-02-10T23:13:01Z

   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
   
   
   
   
   

2. Re: Postgres IO sweet spot

   rstander@exa.co.za — 2026-02-16T14:57:14Z

   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
   >
   >
   >
   

3. Re: Postgres IO sweet spot

   Greg Sabino Mullane <htamfids@gmail.com> — 2026-02-16T15:34:47Z

   On Tue, Feb 10, 2026 at 6:13 PM Riaan Stander <rstander@exa.co.za> wrote:
   
   > 3 x Host Servers running Microsoft Storage Spaces in a 3 way mirror
   >
   
   That's an expensive way to provide some HA. What's the business
   requirement? How does that tie into Postgres? Might be able to do it in
   other ways.
   
   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.
   >
   
   Yikes! Yes, SSD would be a big win. It's orders of magnitude faster, and
   just removes so many problems.
   
   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.
   
   
   Sorry, I have no numbers to provide you there, but I cannot imagine any
   amount of tuning is going to be as big a win as going to SSD.
   
   Cheers,
   Greg
   
   --
   Crunchy Data - https://www.crunchydata.com
   Enterprise Postgres Software Products & Tech Support
   

4. Re: Postgres IO sweet spot

   rstander@exa.co.za — 2026-02-17T23:42:25Z

   > That's an expensive way to provide some HA. What's the business 
   > requirement? How does that tie into Postgres? Might be able to do it 
   > in other ways.
   We used to run a SAN shared between our host servers, but this was 
   replaced with Storage Spaces. I think they don't trust Postgres native 
   HA capabilities and want some hardware guarantee.
   
   > Yikes! Yes, SSD would be a big win. It's orders of magnitude faster, 
   > and just removes so many problems.
   I assume it will help, but I fear however that the overhead with a 3 way 
   mirror is not going to be solved with just adding SSD. I'm trying to get 
   them to rather deploy direct attached NVME/SSD to each Host and then use 
   PG HA from there.
   
   > Sorry, I have no numbers to provide you there, but I cannot imagine 
   > any amount of tuning is going to be as big a win as going to SSD.
   It does take a lot of convincing and arguing though, so concrete number 
   help get the point across.
   
   Thanks for the response
   

5. Re: Postgres IO sweet spot

   Merlin Moncure <mmoncure@gmail.com> — 2026-03-10T23:07:30Z

   On Tue, Feb 17, 2026 at 4:42 PM Riaan Stander <rstander@exa.co.za> wrote:
   
   > That's an expensive way to provide some HA. What's the business
   > requirement? How does that tie into Postgres? Might be able to do it in
   > other ways.
   >
   > We used to run a SAN shared between our host servers, but this was
   > replaced with Storage Spaces. I think they don't trust Postgres native HA
   > capabilities and want some hardware guarantee.
   >
   > Yikes! Yes, SSD would be a big win. It's orders of magnitude faster, and
   > just removes so many problems.
   >
   > I assume it will help, but I fear however that the overhead with a 3 way
   > mirror is not going to be solved with just adding SSD. I'm trying to get
   > them to rather deploy direct attached NVME/SSD to each Host and then use PG
   > HA from there.
   >
   > Sorry, I have no numbers to provide you there, but I cannot imagine any
   > amount of tuning is going to be as big a win as going to SSD.
   >
   > It does take a lot of convincing and arguing though, so concrete number
   > help get the point across.
   >
   > Thanks for the response
   >
   
   Spinning disks+cache  was the most common configuration before SSD came
   along.  Burst performance is great but if you overwhelm the cache, write
   performance can fall off a cliff.  This sounds like exactly what is
   happening to you; moving backups off just bought you some time.  Direct
   attached SSD will completely smoke your current setup.
   
   > I think they don't trust Postgres native HA capabilities and want some
   hardware guarantee.
   
   What is this, 2005?  Properly configured HS/SR setups are incredibly robust
   and are the default configuration for amazon RDS and many, many other
   platforms.  Reading between the lines here, it sounds like your storage
   team bought overpriced garbage and is refusing to admit it's not getting
   the job done.  Postgres failover gets tested routinely across a vast array
   of systems, how many times has your exact configuration been tested?
   
   Is your storage shared with other systems?  Do you have any pgbench numbers
   for reference?  What are your commit rates?  (see xact_commit in
   pg_stat_database, tracked over time)
   
   merlin