dont-use-fsync real world impact

[Andrew Bell]

You made me curious, so I just checked my lease database.  16.7 MB, 57,5712
entries currently.

On Sat, Sep 7, 2019 at 2:17 PM Jure Sah <e at juresah.si> wrote:

> Hi,
>
> The documentation clearly states that using the dont-use-fsync option is
> not recommended.
>
> I am wondering what is the realistic impact of this? As I understand the
> kernel commits dirty pages to disk every 30 seconds by default, and this
> is configurable. Wouldn't this mean that at worst 30 seconds worth of
> leases are lost?
>
> The leases file is in most cases relatively tiny (under 1 MB), and could
> easily fit in system storage cache. However the fact that it gets
> fsynced on every commit means that the performance of the DHCP server is
> capped at whatever the performance of the physical storage is. While
> fast storage options exist, this doesn't make the DHCP server
> future-proof as a solution, which is a problem.
>
> From the past correspondence from the mailing list archive I surmise
> that people usually work around this by using hardware cache that does
> not obey fsync, which simply offloads the problem from the kernel to the
> cache controller and only superficially solves the problem. It however
> hints towards the view that perhaps not using fsync is not all that bad,
> if we are talking about a typical professionally hosted server.
>
> LP,
> Jure
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