[kea-dev] On persistent lease file clean-up mechanism

Marcin Siodelski marcin at isc.org
Mon Nov 24 15:38:25 UTC 2014


Nicolas,

Thanks for the write-up. In general, I feel it is a right direction. But
I also agree it is not a trivial matter as the solution should not
impair the DHCP service while the clean-up is in place. Also, the
solution should prevent race conditions and data loss.

As a result of the discussions on the mail list we will have to create a
page on the trac wiki which will contain requirements for this feature
as well as some little design. I will make sure such page is created
once we have first thoughts exchanged.

We haven't yet gone through the phase of discussing this feature because
it was somewhat out of scope for the 0.9.1 release. So, all my thoughts
here are preliminary and I may be wrong.

I think we should consider other triggers for the clean-up apart from
the timer. Though, the timer could be the first one to implement and
other could wait. We could consider triggering clean-up when the lease
expiration counter is X, or number of renewed leases is Y etc.

I agree that the clean-up should be performed on a renamed file and the
server should use a different/empty file for new lease updates while the
clean-up is in place. However, there is a question how the data is
combined when the clean-up is done. Keeping the compressed (cleaned-up)
file separately from the currently used file is an option, but it means
that each clean-up operation results in creation of one new file holding
partial information about the leases. I believe that in many cases
administrators would rather want lease information be stored in a single
file, not multiple. Also, the subsequent clean-up operations would need
to process all saved lease files, since each of them potentially holds
some lease information which may have expired.

I tend to think that the lease data aggregation should rather take place
at the end of the clean-up phase and should result in having at most two
files: one currently used by the server to write lease updates until the
next clean-up is triggered. Another one, holding all remaining
(historical) and cleaned-up lease information. Ideally, they should be
combined into a single file, but appending the contents of the currently
used file may have some impact on the service availability for a period
of time when the append is being done.

So, the option 1 would be...
When the clean-up is triggered, the lease file used currently by the
server is renamed. The new file is created for the server to use. The
renamed file contents are appended to the lease file holding historical
data. Then, clean-up is performed on this file with appended
information. The server keeps using the same lease file (other than the
one on which the clean-up was performed) until next clean-up comes.

And option 2 ...
There is only one lease file. When the clean-up is triggered this file
is renamed and the new file is created for the server to write new lease
updates. When the clean-up is completed on the renamed lease file, the
DHCP service is ceased for a short period of time when the most recent
lease updates (gathered during clean-up) are appended to the cleaned-up
file and the server switches to use this file.

Both solutions are similar, but second option results in use of a single
lease file (which I personally prefer). The first option has an
advantage that there is no need to copy over the most recent information
to the single lease file.

I am not sure I fully understand the proposal of keeping the clean-up
interval "at least equal to maximum lease time value". Are you proposing
that clean-ups are triggered no more frequently than maximum
valid-lifetime that may occur for any lease in a lease file? Why server
restart is associated with the lease file cleanup in this context? The
clean-up should not be triggered until the server starts up and loads
lease information from the existing lease files, at which point the
server records all updates to leases in an available lease file. And the
clean-up should ensure that the server always has a lease file to write to.

Marcin


On 11/19/14 15:27, Chaigneau, Nicolas wrote:
> Hello,
> 
> 
> I'd like to discuss the topic of cleaning-up the lease file in the case of a "memfile" back-end.
> That's probably something you've already thought of, but I didn't find specific implementation discussed of on the mailing list archive.
> Since it's still yet to be implemented, I'd like to share my 2 cents on the subject.
> 
> This is not a trivial matter: in the context of high availability and very large lease files involved, the situation of a server being unresponsive for several seconds while it handles rewriting the lease file is not something we can afford.
> 
> So here's how I would do it:
> - define a lease file clean-up interval; for instance 1H.
> - every 1H, trigger the clean-up mechanism:
>     - rename <lease file> to <old lease file> (<lease file>~ or any other naming convention)
>     - recreate an empty <lease file>
>     - close and reopen Kea's file handle on <lease file>
> - at server startup, load in memory both <lease file> and <old lease file> (aggregating their content)
> 
> If we ensure that the clean-up interval value is *at least* equal to the maximum lease time value, this guarantee no data can be lost during a server restart: any lease not expired is necessarily either in the current or old lease file (possibly both).
> 
> Both <lease file> and <old lease file> being in the same directory, hence on the same filesystem, the rename operation is immediate. The file's inode and content are unchanged. Only the file name is modified.
> 
> 
> This probably would not be suitable for everyone's needs, so maybe this could be an optional mechanism.
> 
> 
> What do you think ?
> 
> 
> Regards,
> Nicolas.
> 
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