Hi Alex,
On Sun, Sep 07, 2025 at 10:32:52AM +0200, Aleksandar Lazic wrote:
> Hi Willy.
>
> On 2025-09-05 (Fr.) 16:47, Willy Tarreau wrote:
> > Hi all,
> >
> > I've got a report of consistent hash delivering different hashes since 3.0
> > with
> > commit faa8c3e02 ("MEDIUM: lb-chash: Deterministic node hashes based on
> > server
> > address").
> >
> > The cause is a mistake in the ID-based key calculation (the hash is applied
> > twice and the ID range scaling was dropped). The fix is trivial:
> >
> > --- a/src/lb_chash.c
> > +++ b/src/lb_chash.c
> > @@ -123,7 +123,7 @@ static inline u32 chash_compute_server_key(struct
> > server *s)
> > case SRV_HASH_KEY_ID:
> > default:
> > - key = full_hash(s->puid);
> > + key = s->puid * SRV_EWGHT_RANGE;
> > break;
> > }
>
> For which directive/config can I grep to verify if the consistent hash is in
> use?
Good point, I forgot the most important: you're affected if you're in 3.0+
with "hash-type consistent" and no "hash-key" (or "hash-key id") setting in
the same backend which delivers traffic to cache nodes. And if you have them
but upgraded from 2.9 or older without noticing, it means that your load
doesn't have an impact on the caches.
Thanks,
Willy
