Hey fellow xen''ers - We are going to be setting up a new Xen server, my first, and I have a question. From what I have gathered reading through the documentation, you are limited in the number of domains you can have running by the amount of memory you have. Our server is going to be starting with 4GB of ram. Now, if we want to have domains with 1GB of ram, does this mean we can only have 4 domains? Or is there a more dynamic way of doing this? In other words, something like, "Domain1 can use up to 1GB of ram, Domain2 can use up to 2GB of ram, Domain3 can use up to 4GB of ram". And any time Domain3 needs 4GB, it can have it if it''s free, otherwise it has to swap. Is that even possible? Or is there another solution? Or am I understanding the docs totally wrong? Any help with this is GREATLY appreciated. Thanks! ------------------------------------- Brian Johnson Systems Administrator ThermoAnalytics, Inc. Ph: 906-482-9560 x210 Fax: 906-482-9755 http://www.thermoanalytics.com _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
Petersson, Mats
2005-Sep-23 15:51 UTC
RE: [Xen-users] Number of domains versus memory usage
> -----Original Message----- > From: xen-users-bounces@lists.xensource.com > [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of > Brian Johnson > Sent: 23 September 2005 16:34 > To: xen-users@lists.xensource.com > Subject: [Xen-users] Number of domains versus memory usage > > Hey fellow xen''ers - > > We are going to be setting up a new Xen server, my first, and > I have a question. From what I have gathered reading through > the documentation, you are limited in the number of domains > you can have running by the amount of memory you have. Our > server is going to be starting with 4GB of ram. > > Now, if we want to have domains with 1GB of ram, does this > mean we can only have 4 domains? Or is there a more dynamic > way of doing this? In other words, something like, "Domain1 > can use up to 1GB of ram, Domain2 can use up to 2GB of ram, > Domain3 can use up to 4GB of ram". And any time Domain3 > needs 4GB, it can have it if it''s free, otherwise it has to swap.You can let the Linux within each guest do swapping if you like, you just need to give each one a swap-partition (which is really a file in the Dom0 environment). A guest can then use the amount of memory it''s been granted, and if it needs more it will swap some stuff out to disk and deal with it just the same way that a regular machine does when the total memory load is bigger than the available memory... What isn''t possible is for Xen to allocate/free memory and re-distribute it automatically. Nor can Xen swap pages in or out. So the total memory given to all guests at any given time is always going to be less than the total amount of memory in the system [because Xen itself takes some memory]. If you have 4GB of usuable ram after booting, you can have four guests of 1GB each. Or one guest of 0.5GB, and three of 1.0GB, and two of 0.25GB. If you have 4GB in the machine, however, you''ll need Xen 3.0 to make use of ALL of the RAM, because the BIOS will steal some memory between 0-4GB to use for PCI address space, and that will give a total memory AVAILABLE of about 3.3-3.8GB below 4GB. Xen 2.0 isn''t able to use memory above 4GB. Xen 3.0 is currently in "testing" mode, so it''s not considered fully stable, but OK for testing purposes. So in your example, using Xen 3.0, we''d probably come up with a configuration like this: Domain 1: 900MB Domain 2: 900MB Domain 3: 900MB Domain 4: 900MB That leaves a couple of hundred megabytes for Xen + Dom0, which is needed, because Dom0 is another copy of Linux, and it needs that sort of memory if you want to do anything useful with it. Xen itself uses something around 4-10MB, so not a huge amount. In Xen 2.0 it would be another 100MB or so less on each guest domain. [Dom0 still needs to have the same amount of memory]. Of course, if one of your domains has something extra memory demanding [say a database of several hundred megabytes that you want to keep in memory at all times], then you could shuffle the memory allocation around so that domain can use more memory, and the others get less. [I started writing this using the word Guest, then changed to using the word Domain. They are essentially interchangeable terms in the usage above.] -- Mats _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
Ernst Bachmann
2005-Sep-23 16:09 UTC
Re: [Xen-users] Number of domains versus memory usage
On Friday 23 September 2005 17:33, Brian Johnson wrote:> Hey fellow xen''ers - > > We are going to be setting up a new Xen server, my first, and I have a > question. From what I have gathered reading through the documentation, you > are limited in the number of domains you can have running by the amount of > memory you have. Our server is going to be starting with 4GB of ram. > > Now, if we want to have domains with 1GB of ram, does this mean we can only > have 4 domains?Yes.> Or is there a more dynamic way of doing this? In other > words, something like, "Domain1 can use up to 1GB of ram, Domain2 can use > up to 2GB of ram, Domain3 can use up to 4GB of ram". And any time Domain3 > needs 4GB, it can have it if it''s free, otherwise it has to swap. > > Is that even possible? Or is there another solution? Or am I > understanding the docs totally wrong? Any help with this is GREATLY > appreciated. Thanks! >Well, there''s the "balloon" driver, which marks memory as "used" inside a domain and gives it back to xen, but it doesn''t do so dynamic based on the current domains needs. And a assignment scheme like "give up to 4GB" to domainX won''t work well with linux inside that domain anyways, as linux usually tries to use ALL available ram in some way (disk cache...) so after some time all of your domains would always use the max. ram they''re allowed to. if you need to run more domains on your hardware, but don''t need the level of isolation xen provides (all your planned domains would run linux with the same kernel), have a look at http://linux-vserver.org/. /Ernst _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users