[rrd-developers] rfc: later caching
kbrint at rufus.net
Fri Apr 17 14:34:10 CEST 2009
On Fri, Apr 17, 2009 at 08:38:10AM +0200, Tobias Oetiker wrote:
> disclaimer: this is about 1.5 not 1.4 !
> * the (big) disadvantage is that updatev does not work anymore, and
> for larger deployments updatev is a cornerstone function in
> driving holt winters based alerting.
Tobi, I was already planning updatev support along these lines, although I
didn't have an idea when we'd integrate it (i.e. 1.5 vs 1.4.later).
> * it would require the cached to read the header information of the
> rrdfiles once and cache them internally so that it can calculate
> the updates without accessing the disk, but since header
> information is quite small, a decent sized machine could
> easily keep hundreds of thousands of headers in the cache daemon.
I think the best first approach would be:
- on the first updatev, take an in-memory copy of the live_head and *_prep
- no need to do it for update
- CON: when the daemon starts up, there is heavy read load
- a copy of the header allows us to do input validation on update strings
(i.e. correct ds_cnt)
- split up process_arg() into:
- parse update string, update in-memory pdp/cdp/*_prep
- write the RRAs
I thought we'd do it in stages:
(1) Process the update string twice. Once when updatev received from the
client (update in-memory copy). Once when we call rrd_update_r() with
the update string.
- minimal changes to the current rrdcached code/data structures
- CPU overhead minimal, still large delayed IO benefit of rrdcached
- easiest change that gives updatev support
(2) Process the update string once. Cache the computed results, and write
them out to the RRD after delay.
- requires we re-think rrd_update_r() or create a function to pass
pre-computed values into the RRD
I'm guessing (1) will be significantly easier than (2), and provide almost
the same functionality (if slightly less efficiently).. I think staging
it like this would make the most sense.
kevin brintnall =~ /kbrint at rufus.net/
More information about the rrd-developers