I dug deeper in the code and found out that server health checks also call
dns resolution. The hold.valid was used before the change at
https://github.com/haproxy/haproxy/commit/f50e1ac4442be41ed8b9b7372310d1d068b85b33
to trigger the resolution. After the change it is triggered no more. So the
resolution if driven only by timeout resolve now. In other words hold.valid
has not much meaning.
When a server is in layer 4 connection error there are tasks inserted into
scheduler queue which when executed never do any job after the change. This
is suboptimal but I guess are not many installations with servers in this
state for a long time.
Can anyone confirm my understating of this setting ?
śr., 22 wrz 2021 o 19:52 Michał Pasierb napisał(a):
> I would like to clarify how *hold valid* is used by resolvers. I have
> this configuration:
>
> resolvers mydns
> nameserver dns1 192.168.122.202:53
> accepted_payload_size 8192
>
> timeout resolve 5s
> timeout retry 2s
> resolve_retries 3
>
> hold other 30s
> hold refused120s
> hold nx 30s
> hold timeout10s
> hold valid 1h
> hold obsolete 0s
>
> The *valid* setting is a bit confusing. I can not find good explanation
> of it in the documentation. From what I see in the code of version 2.0.25,
> it is used only when doing DNS queries in tcp/http path:
>
> http-request do-resolve(txn.myip,mydns,ipv4) str(services.example.com)
>
> only when requests arrive in parallel, I see less queries to DNS servers
> than http requests. When requests are done in sequence, I see the same
> count of DNS requests as http requests. For example when I send 3000
> requests to HAProxy with 3 clients in parallel, there are about 2600
> requests to DNS servers.
>
> So it doesn't look like a proper cache to me. Whole HAProxy becomes
> unusable 10 seconds (hold timeout) after DNS servers stop responding
> because every server which is using DNS SRV record is put to maintenance
> state due to resolution error.
>
> Is this proper assessment of current state and is this what was intended ?
>
> Regards,
> Michal
>
