Re: Recommended value for pg_test_fsync

Hi Haroldo,

Thank you for the details.

We are using xfs on IBM Power Linux Rhel7 but I will check this in our
environment and get back to you with the results.

Thanks and Regards,
Nikhil

On Wed, Jul 1, 2020, 22:46 Haroldo Kerry <hkerry(at)callix(dot)com(dot)br> wrote:

> Hello Nikhil,
> We had performance issues with our Dell SC2020 storage in the past. We had
> a 6 SSD RAID10 setup and due all the latencies expected 20K IOPS but were
> getting 2K...
> After *a lot* of work the issue was not with the storage itself but with
> the I/O scheduler of the filesystem (EXT4/Debian 9).
> The default scheduler is CFQ, changing to deadline provided us the 10x
> difference that we were expecting.
> In the end this was buried on the storage documentation that
> somehow slipped us...
> Hope this helps.
> Regards,
> Haroldo Kerry
>
> On Wed, Jul 1, 2020 at 2:06 PM Nikhil Shetty <nikhil(dot)dba04(at)gmail(dot)com>
> wrote:
>
>> Hi Jeff,
>>
>> Thank you for your inputs. We may stick with fdatasync for now. We will
>> get more details on connection details between SAN and server from the
>> storage team and update this thread.
>>
>> Storage is Hitachi G900 with 41Gbps bandwidth.
>>
>> Thanks and regards,
>> Nikhil
>>
>>
>>
>> On Tue, Jun 30, 2020 at 9:51 PM Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>>
>>> On Mon, Jun 29, 2020 at 5:27 AM Nikhil Shetty <nikhil(dot)dba04(at)gmail(dot)com>
>>> wrote:
>>>
>>>> Hi Team,
>>>>
>>>> We have a PostgreSQL 11.5.6 database running on VM.
>>>> RAM - 48GB
>>>> CPU - 6 cores
>>>> Disk - SSD on SAN
>>>>
>>>> We wanted to check how the WAL disk is performing using
>>>> pg_test_fsync.We ran a test and got around 870 ops/sec for opendatasync and
>>>> fdatasync and just 430 ops/sec for fsync.We feel it is quite low as
>>>> compared to what we get for local storage(2000 ops/sec for fsync).
>>>>
>>>
>>> It is not surprising to me that SAN would have higher latency than
>>> internal storage. What kind of connection do you have between your server
>>> and your SAN?
>>>
>>>
>>>> What is the recommended value for fsync ops/sec for PosgreSQL WAL disks
>>>> on SAN ?
>>>>
>>>
>>> You have the hardware you have. You can't change it the same way you
>>> can change a config file entry, so I don't think that "recommended value"
>>> really applies. Is the latency of sync requests a major bottleneck for
>>> your workload? pg_test_fsync can tell you what the latency is, but can't
>>> tell you how much you care.
>>>
>>> Cheers,
>>>
>>> Jeff
>>>
>>>>
>
> --
>
> Haroldo Kerry
>
> CTO/COO
>
> Rua do Rócio, 220, 7° andar, conjunto 72
>
> São Paulo – SP / CEP 04552-000
>
> hkerry(at)callix(dot)com(dot)br
>
> www.callix.com.br
>