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Slow performance adding or deleting zones with large-ish (>480k) zones database

by robm＠fastmail.com

Hi We're noticing that as our list of zones gets larger (about 480k right now), adding a new zone or deleting an existing zone seems to continue to get slower. We are always doing our modifications as part of a transaction, and the time appears to occur in the commit phase. An example timing. # time /opt/knot/sbin/knotc ... conf-begin OK real 0m0.010s user 0m0.000s sys 0m0.010s # time /opt/knot/sbin/knotc ... conf-unset zone.domain example.com OK real 0m0.010s user 0m0.000s sys 0m0.010s # time /opt/knot/sbin/knotc ... conf-commit OK real 0m2.330s user 0m0.000s sys 0m0.009s # As you can see, it took > 2 seconds to commit the transaction that removes just the example.com zone. Similarly, it takes > 2 seconds to commit the transaction that adds the zone back. Given the time is real time and not sys/user, I presume knotc is waiting on knotd to complete the work. I used perf to record a CPU profile of knotd while the commit was running, but nothing hugely stuck out at me. 10.75% knotd libc.so.6 [.] __memcmp_avx2_movbe ◆ 6.03% knotd knotd [.] __popcountdi2 ▒ 5.89% knotd knotd [.] ns_first_leaf ▒ 5.25% knotd libc.so.6 [.] pthread_mutex_lock@@GLIBC_2.2.5 ▒ 3.85% knotd liblmdb.so.0.0.0 [.] 0x0000000000003706 ▒ 3.72% knotd knotd [.] ns_find_branch.part.0 ▒ 2.76% knotd knotd [.] trie_get_try ▒ 2.63% knotd liblmdb.so.0.0.0 [.] 0x00000000000069d2 ▒ 2.34% knotd libknot.so.14.0.0 [.] knot_dname_lf ▒ 1.92% knotd liblmdb.so.0.0.0 [.] mdb_cursor_get ▒ 1.72% knotd knotd [.] create_zonedb ▒ 1.68% knotd knotd [.] twigbit.isra.0 ▒ 1.68% knotd knotd [.] catalogs_generate ▒ 1.36% knotd knotd [.] twigoff.isra.0 ▒ 1.28% knotd knotd [.] hastwig.isra.0 ▒ 1.28% knotd knotd [.] db_code ▒ 1.27% knotd libknot.so.14.0.0 [.] find_item ▒ 1.11% knotd libknot.so.14.0.0 [.] knot_dname_size ▒ 1.04% knotd knotd [.] zonedb_reload ▒ 0.99% knotd libc.so.6 [.] _int_free ▒ 0.99% knotd liblmdb.so.0.0.0 [.] 0x0000000000003ce8 ▒ 0.96% knotd liblmdb.so.0.0.0 [.] memcmp@plt ▒ 0.95% knotd liblmdb.so.0.0.0 [.] mdb_cursor_open ▒ 0.88% knotd libc.so.6 [.] malloc ▒ 0.88% knotd knotd [.] conf_db_get ▒ 0.87% knotd knotd [.] ns_next_leaf ▒ 0.82% knotd libknot.so.14.0.0 [.] iter_set ▒ 0.75% knotd knotd [.] evsched_cancel ▒ 0.73% knotd libknot.so.14.0.0 [.] find ▒ ... Our config is pretty simple, conf-export looks like: server: rundir: "/local/knot_dns/run/" user: "nobody" pidfile: "/local/knot_dns/run/knot.pid" listen: [ ... ] log: - target: "syslog" any: "info" statistics: timer: "10" file: "/tmpfs/knot_dns_stats.yaml" database: storage: "/local/knot_dns/data" mod-stats: - id: "default" request-protocol: "on" server-operation: "on" request-bytes: "on" response-bytes: "on" edns-presence: "on" flag-presence: "on" response-code: "on" request-edns-option: "on" response-edns-option: "on" reply-nodata: "on" query-type: "on" query-size: "on" reply-size: "on" template: - id: "default" global-module: "mod-stats/default" storage: "/local/knot_dns/zones/" zone: - domain: "example.com." template: "default" ... 478,000 more domains all the same ... Current files on disk are: # ls -l /local/knot_dns/data/* /local/knot_dns/data/catalog: total 0 /local/knot_dns/data/journal: total 0 /local/knot_dns/data/keys: total 0 /local/knot_dns/data/timers: total 75880 -rw-rw---- 1 root root 77697024 Jun 24 09:26 data.mdb -rw-rw---- 1 root root 2432 Jul 17 01:05 lock.mdb /local/knot_dns/data/timing: total 0 This machine is not slow or constrained in any way. It's 24 core, 3.6Ghz, 64Gb, NVMe drives, etc. Load is very low (<1) with plenty of free resources. So what I'm wondering is: 1. Is this normal? It doesn't feel right that adding/removing a single domain takes > 2 seconds regardless of the size of the existing zone database 2. Is there any way to improve this? Doing multiple adds/deletes at once within a transaction works and we do that where we can, but there are cases where we can't do that and I'd really like to understand why this is as slow as it is. Thanks in advance Rob

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knot-dns-users Červenec 2025