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    DNS queries during backup job

    Scheduled Pinned Locked Moved Xen Orchestra
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    • ronivayR Offline
      ronivay Top contributor @julien-f
      last edited by

      I'll put this to test and see tomorrow what the DNS query stats look like.

      Just my two cents but i feel like one shouldn't "fix" a flaw or bad behaviour in application by relying on external dependency to deal with it, especially if it's fixable. Sure using something like nscd in XOA would kinda fix the issue in it but wouldn't possible perf issue etc still exist in node? I'm not competent to review the code so can't say anything about the actual implementation in feature branch.

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      • olivierlambertO Offline
        olivierlambert Vates 🪐 Co-Founder CEO
        last edited by

        It's not trivial to decide where to put that "frontier". XOA is meant to be an entire system, not just with XO code, but also the updater and other things.

        For the DNS thing, I have to admit I don't know yet what's the best practice. I suppose it also depends on where do you want to stop thinking about doing "non-core" features (ie DNS caching) vs doing it internally. Should we also implement other "system" stuff? It's not trivial to answer that 🙂

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        • ronivayR Offline
          ronivay Top contributor @olivierlambert
          last edited by

          I think the main point to focus on here is that XO is doing totally unnecessary DNS queries with excessive frequency. I don't see this as implementing a non-core feature but a fix in the logic how application figures out where to connect and how often. How exactly and what options there are is outside of my knowledge 🙂

          ForzaF 1 Reply Last reply Reply Quote 0
          • ForzaF Online
            Forza @ronivay
            last edited by

            IMHO I don't think applications in general have internal dns caching, but they do rely on system provided functionality. So with that in mind it is sensible to use a system package rather than some fixing inside XO code. Especially considering XO can run on other platforms than XOA.

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            • A Online
              Andrew Top contributor @Forza
              last edited by

              @Forza I agree that the OS is responsible for caching host records. The real question is why is XO doing so many lookups repeatedly. Maybe it is actually a Node problem (in addition to code issues).

              In most applications once a socket is opened to a host it stays open and does not need to do another lookup until it is closed and a new connection is made. If XO or Node is stateless and opens a new connection for each block read/write (or group of blocks) then it may do a lot of lookups. The mass lookups seems to be a sign of a lot of overhead that could be reduced to improve performance.

              Yes, nscd can be a host query (DNS) cache solution (for XO source and XOA) but can the code be improved to reduce overhead and improve general performance?

              Here is a quick MRTG image of DNS requests. You can see when I enabled nscd that caches lookup requests (hint, sunday night):
              dns-requests-week.jpg

              olivierlambertO 1 Reply Last reply Reply Quote 2
              • olivierlambertO Offline
                olivierlambert Vates 🪐 Co-Founder CEO @Andrew
                last edited by

                @Andrew said in DNS queries during backup job:

                If XO or Node is stateless and opens a new connection for each block read/write (or group of blocks) then it may do a lot of lookups. The mass lookups seems to be a sign of a lot of overhead that could be reduced to improve performance.

                I agree that's a good question (for @julien-f I assume)

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                • ronivayR Offline
                  ronivay Top contributor @julien-f
                  last edited by

                  @julien-f this changed the situation from thousands of queries in minutes to no noticeable spike in query graphs during backup job, so huge improvement.

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                  • H Offline
                    hoerup
                    last edited by

                    Although it is nice that there is work arounds for the DNS spikes with either nscd or the in-process DNS cache, i think the DNS spikes are a symptom of a whole different issue.

                    I think we can safely assume that each DNS lookup is corresponding to one attempt at establishing a TCP connection then there is some code somewhere that spawns an awfull lot of short lived connections instead of reusing / pooling them - with all the issues that follows in that area (insufficient ulimit NOFILE, connections in TIME_WAIT/exhausting of client ports etc)

                    julien-fJ 1 Reply Last reply Reply Quote 2
                    • julien-fJ Offline
                      julien-f Vates 🪐 Co-Founder XO Team @hoerup
                      last edited by

                      @hoerup I agree with your analysis, not sure how easy it will be to fix, we'll investigate.

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                      • ronivayR Offline
                        ronivay Top contributor
                        last edited by

                        Did some further testing if amount of DNS queries would correlate to the amount of actual connections made to the host. This doesn't seem to be the case which is even more interesting 🙂 Some results below.

                        Ran an incremental from delta backup which took in total of 9 minutes:

                        • Amount of DNS queries: close to 7k
                        • Amount of HTTPS connects logged to host IP-address: 478.
                        • Amount of HTTPS connects/disconnects logged in total to host IP-address: 955

                        Connection counts were about the same with installation from dns.lookup branch provided by @julien-f above, without the amount of DNS queries obviously.

                        ForzaF 1 Reply Last reply Reply Quote 2
                        • ForzaF Online
                          Forza @ronivay
                          last edited by

                          @ronivay are all dns queries for the same host and record?

                          ronivayR 1 Reply Last reply Reply Quote 0
                          • ronivayR Offline
                            ronivay Top contributor @Forza
                            last edited by ronivay

                            Yep. Same domain, asks A and AAAA at the same time, both being individual queries obviously.

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                            • olivierlambertO Offline
                              olivierlambert Vates 🪐 Co-Founder CEO
                              last edited by

                              Also XAPI (so on host side) doesn't support HTTP/2.

                              julien-fJ 1 Reply Last reply Reply Quote 0
                              • julien-fJ Offline
                                julien-f Vates 🪐 Co-Founder XO Team @olivierlambert
                                last edited by

                                The DNS cache has been merged, keep us posted if you have any issues. 🙂

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