[Samba] gencache.tdb size and cache flush

[Francesco Malvezzi]

> 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

thank you so much for sharing your experience. The daily restart task
has no drawbacks and it is wise.

Your environment is really awesome: compliments for the tuning work you
did, that is surely the hardest part of the job.

thank you,

franz