[Samba] gencache.tdb size and cache flush

Peter Eriksson peter at ifm.liu.se
Wed Aug 29 13:36:23 UTC 2018

For what it’s worth you are not alone in seeing similar problems with Samba and gencache. 

Our site has some 110K users (university with staff & students (including former ones), and currently around 2000 active (SMB) clients connecting to 5 different Samba servers (around 400-500 clients per server). When we previously just let things “run” gencache.tdb would grow forever and authentication login performance would start to deteriorate after a little while (would take more than 10 seconds). So we now delete it (and locks/locking.tdb that also tends to grow forever) and restart our samba processes every morning at 7 am - which gives us much more stable performance.

- Servers with 256GB of RAM, 10Gbps ethernet interfaces and around 110TB of disk per server.
- FreeBSD 11.2-p2
- Samba 4.7.6 with some local patches to allow (much) bigger socket listening queues in order to handle the case of many clients connecting at the same time.

(We are trying to upgrade to a more recent Samba but 4.7.8 and 4.7.9 gave us horrible authentication performance every 10:th hour where the servers basically denied clients to login for about 2 hours so we had to back down to 4.7.6 again).

- Peter

> On 29 Aug 2018, at 10:28, Francesco Malvezzi via samba <samba at lists.samba.org> wrote:
> Hi all,
> I have a midsize AD domain with some 50k users but only 100 workstations
> joined.
> Sometimes I find server CPU throttling at 100%. In order to let it drop
> and have smooth performance I delete cache:
> systemctl stop samba
> net cache flush
> systemctl start samba
> First of all, is it needed a samba stop to flush the cache?
> Even if cache flush does the job to restore performance, I am clueless
> about the root cause of the problem. Before flushing cache the
> gencache.tdb had 15k entries. Is it large? Do you think is it worth time
> to investigate why it grows so much or is it just normal?
> thank you,
> franz
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