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Am Wed, 20 Jan 2016 01:46:29 +0100 |
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schrieb lee <lee@××××××××.de>: |
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|
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> >> Overcommitting disk space sounds like a very bad idea. |
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> >> Overcommitting memory is not possible with xen. |
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> > |
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> > Overcommitting diskspace isn't such a bad idea, considering most |
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> > installs never utilize all the available diskspace. |
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> |
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> When they do not use it anyway, there is no reason to give it to them |
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> in the first place. And when they do use it, how do the VMs handle |
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> the problem that they have plenty disk space available, from their |
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> point of view, while the host which they don't know about doesn't |
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> allow them to use it? |
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> |
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> Besides, overcommitting disk space means to intentionally create a |
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> setup which involves that the host can run out of disk space easily. |
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> That is not something I would want to create for a host which is |
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> required to function reliably. |
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> |
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> And how much do you need to worry about the security of the VMs when |
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> you build in a way for the users to bring the whole machine, or at |
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> least random VMs, down by using the disk space which has been |
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> assigned to them? The users are somewhat likely to do that even |
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> unintentionally, the more the more you overcommit. |
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|
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Overcommitting storage is for setups where it's easy to add storage |
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pools when needed, like virtual SAN. You just monitor available space |
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and when it falls below a threshold, just add more to the storage pool |
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whose filesystem will grow. |
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|
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You just overcommit to whatever storage requirments you may ever need |
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combined over all VMs but you initially only buy what you need to start |
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with including short term expected growth. |
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|
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Then start with clones/snapshots from the same VM image (SANs provide |
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that so you actually do not have to care about snapshot dependencies |
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within your virtualization software). |
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|
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SANs usually also provide deduplication and compression, so at any |
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point you can coalesce the images back into smaller storage |
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requirements. |
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|
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A sane virtualization solution also provides RAM deduplication and |
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compaction so that you can overcommit RAM the same way as storage. Of |
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course it will at some point borrow RAM from swap space. Usually you |
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will then just migrate one VM to some other hardware - even while it is |
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running. If connected to a SAN this means: You don't have to move the |
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VM images itself. The migration is almost instant: The old VM host acts |
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as some sort of virtualized swap file holding the complete RAM, the new |
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host just "swaps in" needed RAM blocks over network and migrates the |
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rest during idle time in the background. This can even be automated by |
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monitoring the resources and let the VM manager decide and act. |
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|
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The Linux kernel lately gained support for all this so you could |
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probably even home-brew it. |
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|
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-- |
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Regards, |
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Kai |
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|
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Replies to list-only preferred. |