Thorsten von Eicken wrote:
- I'm wondering how we could overcome the RRD working set issue. Even with rrdcached and long cache periods (e.g. I use 1 hour) it seems that the system comes to a crawl if the RRD working set exceeds memory. One idea that came to mind is to use the caching in rrdcached to convert the random small writes that are typical for RRDs to more of a sequential access pattern. If we could tweak the RRD creation and the cache write-back algorithm such that RRDs are always accessed in the same order, and we manage to get the RRDs allocated on disk in that order, then we could use the cache to essentially do one sweep through the disk per cache flush period (e.g. per hour in my case). Of course on-demand flushes and other things would interrupt this sweep, but the bulk of accesses could end up being more or less sequential. I believe that doing the cache write-back in a specific order is not too difficult, what I'm not sure of is how to make it such that the RRD files get allocated on disk in the that order too. Any thoughts?One further thought, instead of trying to allocate RRDs sequentially, if there is a way to query/detect where each RRD file is allocated on disk, then rrdcached could sort the dirty tree nodes by disk location and write them in that order. I don't know whether Linux (or FreeBSD) have a way to query disk location or to at least infer it. TvE |
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