Hello Thomas,
Welcome to Communities! Some useful info on participating here:
VMware Community Code of Conduct
The first thing that needs to be established is: how much is persistent-logging worth to you/customer here?
"* I know that it's totaly forbiden to save logs directly on vsan Storage."
Correct, don't do this.
Also some controllers have limitations of not functioning well when you have vSAN and VMFS on the same controller even when the VMFS is just used for logging e.g. H730P controllers - you should be avoiding these anyway, get HBA330.
VMware Knowledge Base
"* Not possible to think about adding persistent storage on server."
Why? Are you using all of the disk-slots already? I don't see how this would be possible with only 2xDG per node even if using 3.5 inch drives, sure all front-accessible slots maybe, but from the spec-sheet of R740xd these appear to have more than just these.
http://topics-cdn.dell.com/pdf/poweredge-r740xd_reference%20guide3_en-us.pdf
Do these nodes come with a SATADOM option? I ask as I can only find this option for 630/730 etc.
If so, are these reasonably priced or exorbitant?
"* I have no NFS share or Syslog servers available outside of my vsan infrastructure."
How much would a small NFS/syslog set-up cost to buy/steal/borrow and/or is this a no-go for other reasons? I can see that multi-site might make this problematic also.
"What do you think about creating 2 VMs, one per fault domain, hosting them on the VSAN itself."
This is essentially just logging to vSAN but through another potentially problematic nested scenario - this isn't going to "bypass that issue" and really could just add more problems.
"Also, could someone tell me what happens if NFS share becomes innacessible ?"
It won't be able to save anything there until it comes back - only non-active zipped log files are actually committed to disk really IIRC.
Bob
