XenServer 7 Dom0 memory usage



  • So I just finished setting up a XenServer7 with 2 2012r2 VMs over the weekend and in the last two hours I've become aware of some warnings from Xenserver in regards to low memory for Dom 0. The XenServer host is a Dell R530 with 16GB of RAM. VM1 has 12GB of RAM allocated because it's hosting QuickBooks Enterprise & Intuit recommends 8GB of ram, and I figured another 4GB because it's also the main file server, DC, DNS, DHCP, etc. The Second VM is secondary DC/DNS server and I initially configured it with 2 GB of RAM. I figured this would leave 2GB total for XenServer.

    Eearlier when I become aware of the issue, XenServer was at 98% of it's allocated memory. I adjusted the secondary VM down to 1GB of RAM to see if this would help. It didn't, but I also didn't reboot the XenServer as everyone is working. Possibly rebooting would have resolved the issue.

    Now currently it's at 100.8 percent of it's allocated memory. I'm thinking the plan is to now wait until the end of the day (5 or 6 PM) and adjust the primary VM's memory down to maybe 10GB, and the secondary VM back to 2GB. That should then leave 4GBs free. From XenCenter I can see it's telling me that XenServer is currently using 1.3GB.

    With those adjustments do you think I'll be okay or does it sound like I've got a bigger issue?



  • Sounds like it will be okay. And it sounds like that main VM won't need as much as you have assigned, although watching it for tuning purposes would make sense. But I think this is a solid plan and should at least work for the moment.



  • DC, DNS and DHCP use essentially no memory at all. They basically need nothing more than the base OS memory to run, and the base is already there for your QB installation. So you should be just fine there.



  • @frodooftheshire Dom0 only has 756MB (that might be for 6.5) assigned by default, which can cause these errors at times. You have to get into the boot config file for Dom0 and change the memory allocation in that. Google dom0 memory should give you instructions to get that changed.



  • @scottalanmiller Cool. Just to play it safe maybe I'll take the primary VM down to 8 then. So far it's not even using 2GB of ram yet, but it's also only been up for today.



  • @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @scottalanmiller Cool. Just to play it safe maybe I'll take the primary VM down to 8 then. So far it's not even using 2GB of ram yet, but it's also only been up for today.

    Not a bad idea. Figure out what it really needs and adjust as needed. Unless you have loads of users 8GB is probably pretty good.



  • @travisdh1 I'm somewhat confused about this because I have read it only has 756MB, but I've also read it can dynamically allocate memory. Also, if it's only 756MB why would XenCenter show that it has 1.3 GB allocated?



  • @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @travisdh1 I'm somewhat confused about this because I have read it only has 756MB, but I've also read it can dynamically allocate memory. Also, if it's only 756MB why would XenCenter show that it has 1.3 GB allocated?

    That may be a vestige of XenServer 6.5 I'm remember and not something needing attention in 7. I remember having to go change that for one of my installs.



  • I would certainly turn the memory back down to 8GB on the DC/Intuit Server.

    I would demote and remove the secondary DC. It is a waste of resources.

    It provides zero services that you require and does nothing but take up resources that can be used elsewhere.

    The only need you have for a DC is to provide authentication and for QuickBooks. QuickBooks is running on a DC. So if the DC is down, so is QuickBooks. Thus there is no need for a secondary.

    Providing a 2nd DC is a waste of time and effort in the era of modern virtualization.

    I would have VM and agent level backups of your single DC going to a local NAS and then offsite.

    I say agent level because it is my understanding that XS cannot provide application aware VM level backups. This means that XS has no way to tell the AD database or the QuickBooks database to put themselves in a consistent state prior to the backup snapshot being taken. Without that functionality, a VM level backup is a crapshoot.

    Honestly, I do not know of any Hypervisor with application level access for something like QuickBooks.



  • @JaredBusch Hi Jared,

    Thanks for replying. My thinking behind the secondary DC was in case I had to reboot the primary VM, the secondary VM could at least handle DNS/login requests. As far as backup goes, the primary server is running Storagecraft Shadowprotect SPX with 15 minute continuous incrementals that get sent to a BDR box/cloud. SPX runs at a low level of the OS and doesn't know the difference between a physical & virtual machine, so it's able to make great backups.



  • @JaredBusch said in XenServer 7 Dom0 memory usage:

    I would demote and remove the secondary DC. It is a waste of resources.

    That's a good point, can't believe that I missed that. Doesn't make any sense to have two on the same box.



  • @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @JaredBusch Hi Jared,

    Thanks for replying. My thinking behind the secondary DC was in case I had to reboot the primary VM, the secondary VM could at least handle DNS/login requests. As far as backup goes, the primary server is running Storagecraft Shadowprotect SPX with 15 minute continuous incrementals that get sent to a BDR box/cloud. SPX runs at a low level of the OS and doesn't know the difference between a physical & virtual machine, so it's able to make great backups.

    If you have ShadowProtect, that handles that discussion well.

    Obviously, I am working on assumptions of roles and needs of the employees.

    Why would you ever want to reboot the DC in the middle of working hours? I mean yeah, you can if QB goes south and such, but then people are already not productive, so who cares about missing a few DNS queries during a reboot? The people are already non productive.



  • @frodooftheshire said in XenServer 7 Dom0 memory usage:

    Thanks for replying. My thinking behind the secondary DC was in case I had to reboot the primary VM, the secondary VM could at least handle DNS/login requests.

    How often do you need to authenticate while a system is rebooting? Reboots normally take seconds to maybe a minute and if QB is your only application, what authetnication is there? Desktops can go for months without AD being up and no one will notice.

    DNS is an issue only if you don't use something like Google for your secondary DNS, which I would because if the primary DNS is down you want people to still see the Internet and so you need this regardless if this hardware fails on you.



  • @frodooftheshire said in XenServer 7 Dom0 memory usage:

    As far as backup goes, the primary server is running Storagecraft Shadowprotect SPX with 15 minute continuous incrementals that get sent to a BDR box/cloud. SPX runs at a low level of the OS and doesn't know the difference between a physical & virtual machine, so it's able to make great backups.

    All good there.



  • @scottalanmiller said in XenServer 7 Dom0 memory usage:

    DNS is an issue only if you don't use something like Google for your secondary DNS, which I would because if the primary DNS is down you want people to still see the Internet and so you need this regardless if this hardware fails on you.

    I completely disagree with this. I know it is a common recommendation, but I have seen too many desktops switch to their secondary DNS and then never switch back. Thus breaking their local connectivity after the server has been rebooted.



  • @JaredBusch Out of all the users only about 4 people are working off QuickBooks. The rest of the staff utilize an application that runs off the Cloud, so my thinking was - if there ever was an issue with the primary VM, the majority of staff would be able to continue working.



  • @JaredBusch said in XenServer 7 Dom0 memory usage:

    @scottalanmiller said in XenServer 7 Dom0 memory usage:

    DNS is an issue only if you don't use something like Google for your secondary DNS, which I would because if the primary DNS is down you want people to still see the Internet and so you need this regardless if this hardware fails on you.

    I completely disagree with this. I know it is a common recommendation, but I have seen too many desktops switch to their secondary DNS and then never switch back. Thus breaking their local connectivity after the server has been rebooted.

    Problem there is that if the server goes down, everything goes down (as far as DNS affects it). I've yet to see a machine go to secondary and never go back. But I work with Windows less often than most.



  • @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @JaredBusch Out of all the users only about 4 people are working off QuickBooks. The rest of the staff utilize an application that runs off the Cloud, so my thinking was - if there ever was an issue with the primary VM, the majority of staff would be able to continue working.

    This is solely about DNS, though, right and not about QuickBooks?
    Is there even a reason to have AD for QuickBooks, then?



  • @scottalanmiller I thought it was against best practices to use outside DNS as a secondary? From what I've read, even though it's the secondary, you can have instances where DNS requests will try to go there first?



  • @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @scottalanmiller I thought it was against best practices to use outside DNS as a secondary? From what I've read, even though it's the secondary, you can have instances where DNS requests will try to go there first?

    It is for the reason stated.

    What I personally do in a small business with only a single server is set their router as the secondary and then the router points to the DC first and then google second.

    In my case the router is an Ubiquiti ERL and this works perfectly. Your mileage will vary with other routers.



  • @scottalanmiller said in XenServer 7 Dom0 memory usage:

    @JaredBusch said in XenServer 7 Dom0 memory usage:

    @scottalanmiller said in XenServer 7 Dom0 memory usage:

    DNS is an issue only if you don't use something like Google for your secondary DNS, which I would because if the primary DNS is down you want people to still see the Internet and so you need this regardless if this hardware fails on you.

    I completely disagree with this. I know it is a common recommendation, but I have seen too many desktops switch to their secondary DNS and then never switch back. Thus breaking their local connectivity after the server has been rebooted.

    Problem there is that if the server goes down, everything goes down (as far as DNS affects it). I've yet to see a machine go to secondary and never go back. But I work with Windows less often than most.

    Yes, that is the common problem, but I have watched it happen too many times over the years. See my previosu post for my modern resolution.



  • @JaredBusch said in XenServer 7 Dom0 memory usage:

    @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @scottalanmiller I thought it was against best practices to use outside DNS as a secondary? From what I've read, even though it's the secondary, you can have instances where DNS requests will try to go there first?

    It is for the reason stated.

    What I personally do in a small business with only a single server is set their router as the secondary and then the router points to the DC first and then google second.

    In my case the router is an Ubiquiti ERL and this works perfectly. Your mileage will vary with other routers.

    That makes sense.



  • @scottalanmiller said in XenServer 7 Dom0 memory usage:

    @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @JaredBusch Out of all the users only about 4 people are working off QuickBooks. The rest of the staff utilize an application that runs off the Cloud, so my thinking was - if there ever was an issue with the primary VM, the majority of staff would be able to continue working.

    This is solely about DNS, though, right and not about QuickBooks?
    Is there even a reason to have AD for QuickBooks, then?

    I'm not sure I'm following you on your last question. They don't have AD for QuickBooks. They have AD to help manage users/computers/group policy/etc. This company has aprox 20 computers/users.



  • @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @scottalanmiller said in XenServer 7 Dom0 memory usage:

    @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @JaredBusch Out of all the users only about 4 people are working off QuickBooks. The rest of the staff utilize an application that runs off the Cloud, so my thinking was - if there ever was an issue with the primary VM, the majority of staff would be able to continue working.

    This is solely about DNS, though, right and not about QuickBooks?
    Is there even a reason to have AD for QuickBooks, then?

    I'm not sure I'm following you on your last question. They don't have AD for QuickBooks. They have AD to help manage users/computers/group policy/etc. This company has aprox 20 computers/users.

    Right, but his point is that you do not care about a DC being online for those services, only for DNS. Those services generally work without the DC being online because they are cached.

    DNS is the only service that is really critical.



  • @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @scottalanmiller said in XenServer 7 Dom0 memory usage:

    @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @JaredBusch Out of all the users only about 4 people are working off QuickBooks. The rest of the staff utilize an application that runs off the Cloud, so my thinking was - if there ever was an issue with the primary VM, the majority of staff would be able to continue working.

    This is solely about DNS, though, right and not about QuickBooks?
    Is there even a reason to have AD for QuickBooks, then?

    I'm not sure I'm following you on your last question. They don't have AD for QuickBooks. They have AD to help manage users/computers/group policy/etc. This company has aprox 20 computers/users.

    THen no need to worry about a failover, if you aren't using it for QB, no one will notice if it is down.



  • @JaredBusch I see what you mean. I initially wasn't even planning on having a secondary VM at all. At the last minute I figured in a pinch it could be helpful if for some reason I had to reboot the primary VM during business hours and users were trying to access their cloud services. And if I was going to have DNS on a secondary server I can't think of a reason to not do DC, because then you get the AD integration, and DNS can read/write instead of just read.



  • @frodooftheshire said in XenServer 7 Dom0 memory usage:

    @JaredBusch I see what you mean. I initially wasn't even planning on having a secondary VM at all. At the last minute I figured in a pinch it could be helpful if for some reason I had to reboot the primary VM during business hours and users were trying to access their cloud services. And if I was going to have DNS on a secondary server I can't think of a reason to not do DC, because then you get the AD integration, and DNS can read/write instead of just read.

    Yeah, that is a perfectly valid reasoning. To me that is more overhead than I want to deal with, thus using the router as a secondary.



  • Just in case you want to adjust the dom0 memory.

    How to Configure dom0 Memory in XenServer 6.1 and Later



  • I agree with @JaredBusch reasoning for not pointing at DC's only in DNS, though his newly proposed solution is pretty cool - assuming the ERL will flip back to the primary as soon as it's available.

    As for the secondary DNS server, assuming you have a standard Windows license, it's really kinda six of one, half dozen of another if you should setup a second VM as an additional DC. I see both sides, can't say that one way or the other is really better. I suppose it's a matter of resources.


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