rsyslog not keeping up with dhcpd

[Alex Moen]

As stated in the original post, I have checked the disk I/O and there is 
no issue there, nor with any other hardware.  I have already posted the 
fix, and it is based on default settings in rsyslog to limit logging. 
IMHO, that is a very poor decision, as any kind of rate limiting 
decisions should be made by the system administrator and not the 
software developer.  I have applied the configuration changes and all 
the DHCPd logs are being written to disk now.

Thanks,

Alex


On 04/15/2016 12:14 PM, John Wobus wrote:
> On Apr 8, 2016, at 4:33 PM, dave c <dhcp at gvtc.drakkar.org> wrote:
>> In addition to the likely rate limiting, I've seen where disk write contention between the leases file and the local syslog can cause issues. Which is why I only send my DHCP logs to an external pair of log servers. This allows me to optimize my log servers for nothing but log appending and have enough storage to keep my logs for 13+ months. Meanwhile, the DHCP servers are purring along and only worrying about minor incidental local logging and the leases file.
>>
>> Dave
>
> +1
>
> If syslog is having problems, your dhcp activity may be reaching the point where disk synchs
> affect dhcpd performance.  Linux commands that show disk activity can show the issue
> ("why is the system with very light cpu activity doing huge numbers of disk synchs?”).
> DHCP’s retries can hide a growing problem, which might cause a trouble that is non-linearly
> related to load: no complaints, then suddenly many.
>
> Logging via udp, or logging without synching eliminates some disk load.  For the lease file
> writes, there are smart controllers with NV caching, there are solid state disks, there is a
> dhcpd feature to batch disk writes (which may be ready for prime-time), or there is
> ramdisk, which unfortunately seriously affects the resiliency of the dhcp service if the OS dies.
> The dhcp load tester, dhcperf reveals a lot.
>
> John Wobus
> Cornell University IT
>
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-- 
Alex Moen
NSTII
North Dakota Telephone Company
701-662-6481