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From: Pierre Barre <[email protected]>
To: Jeff Ross <[email protected]>
To: [email protected]
Subject: Re: PostgreSQL on S3-backed Block Storage with Near-Local Performance
Date: Fri, 25 Jul 2025 11:25:48 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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	<[email protected]>
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Hi,

I went ahead and did that test.

Here is the postgresql config I used for reference (note the wal options (recycle, init_zero) as well as full_page_writes = off, because ZeroFS cannot have torn writes by design).

https://gist.github.com/Barre/8d68f0d00446389998a31f4e60f3276d

Test was running on Azure with Standard D16ads v5 (16 vcpus, 64 GiB memory)

This time, I didn't run ZFS with L2ARC, I just mounted ZeroFS with 9p.

synchronous_commit = off 

postgres@zerofs:~$ pgbench -vvv -c 100 -j 40 -t 1000 bench
pgbench (16.9 (Ubuntu 16.9-0ubuntu0.24.04.1))
starting vacuum...end.
starting vacuum pgbench_accounts...end.
transaction type: <builtin: TPC-B (sort of)>
scaling factor: 50
query mode: simple
number of clients: 100
number of threads: 40
maximum number of tries: 1
number of transactions per client: 1000
number of transactions actually processed: 100000/100000
number of failed transactions: 0 (0.000%)
latency average = 6.239 ms
initial connection time = 68.922 ms
tps = 16026.940646 (without initial connection time)


synchronous_commit = on

postgres@zerofs:~$ pgbench -vvv -c 50 -j 15 -t 1000 bench
pgbench (16.9 (Ubuntu 16.9-0ubuntu0.24.04.1))
starting vacuum...end.
starting vacuum pgbench_accounts...end.
transaction type: <builtin: TPC-B (sort of)>
scaling factor: 50
query mode: simple
number of clients: 50
number of threads: 15
maximum number of tries: 1
number of transactions per client: 1000
number of transactions actually processed: 50000/50000
number of failed transactions: 0 (0.000%)
latency average = 197.723 ms
initial connection time = 46.089 ms
tps = 252.878721 (without initial connection time)


Not great barebones with with synchronous_commit, but still usable!

Best,
Pierre

On Fri, Jul 25, 2025, at 00:44, Pierre Barre wrote:
>> This then begs the obvious question of how fast is this with 
>> synchronous_commit = on?
>
> Probably not awful, especially with commit_delay.
>
> I'll try that and report back.
>
> Best,
> Pierre
>
> On Fri, Jul 25, 2025, at 00:03, Jeff Ross wrote:
>> On 7/24/25 13:50, Pierre Barre wrote:
>>
>>> It’s not “safe” or “unsafe”, there’s mountains of valid workloads which don’t require synchronous_commit. Synchronous_commit don’t make your system automatically safe either, and if that’s a requirement, there’s many workarounds, as you suggested, it certainly doesn’t make the setup useless.
>>>
>>> Best,
>>> Pierre
>>>
>>> On Thu, Jul 24, 2025, at 21:44, Nico Williams wrote:
>>>> On Fri, Jul 18, 2025 at 12:57:39PM +0200, Pierre Barre wrote:
>>>>> - Postgres configured accordingly memory-wise as well as with
>>>>>    synchronous_commit = off, wal_init_zero = off and wal_recycle = off.
>>>> Bingo.  That's why it's fast (synchronous_commit = off).  It's also why
>>>> it's not safe _unless_ you have a local, fast, persistent ZIL device
>>>> (which I assume you don't).
>>>>
>>>> Nico
>>>> --
>> This then begs the obvious question of how fast is this with 
>> synchronous_commit = on?

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