zasto je fizicki MySQL hosting bolji od VM-a?

agvozden: Sve se to vrtelo na XEN-u i nije bilo nikakvih problema.

bogdan.kecman: drbd ima ozbiljne limite
seljenje virtualne masine sa servera na server sa bazom mozes "da zaboravis" da radi automatski, a ako vec moras da pravis skript koji ce da radi failover isto je da li je na vm-u ili na realnoj masini .... dakle vm ti tu opet ne daje nikakav benefit samo taj ha u stvari postaje kompleksniji za odrzavanje

The next step people go to is running MySQL inside a virtual machine. You are again screwing yourself on IOPs. Every VM on the box can now fight each other for a limited number of sync operations to make the data safe on disk. Forget if group_commit works for your MySQL version, having many VMs running MySQL on the same physical box will screw you much more than lack of group_commit ever will. You can probably kiss consistent performance and latency goodbye too (this will largely depend on how VMs are being run by your hosting provider).

The best way to get screwed is to get “free” extra CPU cycles and IOPs that are excess on the physical machine and then to suddenly switch to not getting any “free” ones and instead only the ones you pay for… wonder why your site is suddenly slow to respond where the number of visitors is the same and you’ve changed NOTHING?

Even running MySQL inside a VM that is the only VM on the box has a performance impact. You want to be using each physical machine to its fullest. If you’ve got a bunch of MySQL servers running inside VMs – you are not doing that.

(you can substitute just about any other database server for “MySQL” in all of the above… interestingly enough I have been told that a certain proprietary database server has a very low performance drop when run inside a VM)