We are at the stage, finally, where we are prepared to deploy public facing VM guests. Now we have to answer these questions: How many guests in total should we contemplate and what services are placed on which guests?
My question comes down to whether it is considered advisable to run the primary DNS and IMAP, or indeed all services, on separate vm guests; or should we continue, more or less, with the present split and just move everything into guests on a one-for-one basis from our existing hosts? Will additional vms necessarily increase the amount of time given to system maintenance as I suspect?
Previously, with only a few physical hosts, the number of platforms was fixed and services were split more or less on the basis of internal and external users. Not to mention which host had more available resources at the time the service was implemented.
For example, presently our primary DNS and the IMAP services runs on one server with MailScanner controlled Sendmail used only for local delivery and forwarding. On another host we run the publicly accessible MX MTA and a secondary public DNS. On a third we run our fax server and public web site together with a caching only DNS service.
I very much would appreciate any relevant comments from people who have already resolved this matter together with their reasoning.
On 11/14/2011 10:58 AM, James B. Byrne wrote:
We are at the stage, finally, where we are prepared to deploy public facing VM guests. Now we have to answer these questions: How many guests in total should we contemplate and what services are placed on which guests?
My question comes down to whether it is considered advisable to run the primary DNS and IMAP, or indeed all services, on separate vm guests; or should we continue, more or less, with the present split and just move everything into guests on a one-for-one basis from our existing hosts? Will additional vms necessarily increase the amount of time given to system maintenance as I suspect?
Previously, with only a few physical hosts, the number of platforms was fixed and services were split more or less on the basis of internal and external users. Not to mention which host had more available resources at the time the service was implemented.
For example, presently our primary DNS and the IMAP services runs on one server with MailScanner controlled Sendmail used only for local delivery and forwarding. On another host we run the publicly accessible MX MTA and a secondary public DNS. On a third we run our fax server and public web site together with a caching only DNS service.
I very much would appreciate any relevant comments from people who have already resolved this matter together with their reasoning.
My experience is that the limiting factor is disk I/O. If you're using traditional platter drives, the VMs will span physically differing sections of the disk(s). If the different VMs hit the disk at the same time, the read/write heads will start flying all over the disks and the seek latency will go up. This can quickly degrade performance to an unusable level.
If you use SSDs, this issue goes away but then you need to pay special attention that your host OS properly uses the SSDs (TRIM, garbage collection, etc).
As for processing power; I used to pin VMs to CPU cores. I've since abandoned this as I found the scheduler to do a great job and pushing loads around cores to get best performance.
hth
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Digimer -
James B. Byrne