[knot-dns-users] Knot 1.6.1 and journal size tuning

Marek Vavruša marek.vavrusa at nic.cz
Mon Dec 15 12:59:31 CET 2014


Hi Anand,

In general, transfer size is roughly equal to actual stored size with
the addition of file header and entry descriptors.
That being said, there's some overhead per stored RR (~16B per record,
or so CC @jan.kadlec) since the RRs aren't merged into RRSets to keep
ordering and such.
But as a rule of thumb, you should allow at least twice the desired
maximum transfer size for a journal size for two reasons:
- the journal will be able to keep history of at least one transfer back
- won't need to flush the zone file after each change (helps to avoid
disk usage spikes)

Best,
Marek


On 15 December 2014 at 11:16, Anand Buddhdev <anandb at ripe.net> wrote:
> Hi Knot developers,
>
> I've now installed version 1.6.1 on some servers, and I'm observing some
> journal related issues, and I have questions about them. First off,
> here's one issue:
>
> 2014-12-15T06:00:56 info: [203.in-addr.arpa] NOTIFY, incoming,
> 193.0.0.198 at 53535: received serial 3006121318
> 2014-12-15T06:00:56 info: [203.in-addr.arpa] refresh, outgoing,
> 193.0.0.198 at 53: master has newer serial 3006121317 -> 3006121318
> 2014-12-15T06:00:56 info: [203.in-addr.arpa] IXFR, incoming,
> 193.0.0.198 at 53: starting
> 2014-12-15T06:00:57 warning: [203.in-addr.arpa] IXFR, incoming,
> 193.0.0.198 at 53: failed to write changes to journal (not enough space
> provided)
> 2014-12-15T06:00:58 notice: [203.in-addr.arpa] IXFR, incoming,
> 193.0.0.198 at 53: fallback to AXFR
> 2014-12-15T06:00:58 info: [203.in-addr.arpa] AXFR, incoming,
> 193.0.0.198 at 53: starting
> 2014-12-15T06:00:59 info: [203.in-addr.arpa] AXFR, incoming,
> 193.0.0.198 at 53: finished, serial 3006121317 -> 3006121318, 0.65 seconds,
> 171 messages, 7960312 bytes
>
> So it looks like the IXFR is too big, and won't fit into the journal,
> and Knot is falling back to AXFR. When I requested this IXFR by hand, I got:
>
> $ dig ixfr=3006121317 203.in-addr.arpa @193.0.0.198
> ...
> ...
> ;; XFR size: 121779 records (messages 170, bytes 7963389)
>
> The size of the IXFR in bytes is below the configured file size limit
> (10M), but I suspect that 7963389 bytes probably take up more room in
> the journal, so Knot can't write into it, and is falling back to AXFR.
>
> Are you able to tell me (approximately of course), how much disk space
> is required for a given number of bytes of IXFR? This will help me tune
> the setting of ixfr-fslimit to avoid this unnecessary fallback to AXFR.
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