On Fri, Sep 07, 2012 at 08:46:56PM +0000, Edward Ned Harvey (lopser) wrote:
> I believe these products exist, but I'm having difficulty finding
> them. You want to provide HA virtualization... Meaning some VM
> should appear to always be up, even if half of the underlying
> hardware were to die. At first blush, it would seem impossible ...
> Even the fastest network can't possibly keep up with the internal CPU
> state and memory of the guest VM. But I would swear, some years ago,
> I saw or heard something intelligent ... The host OS is able to
> quickly snapshot and diff the guest machine state, so it does this at
> critical moments, like, when the guest OS is sending outbound network
> packets. So you don't actually need to keep the complete guest
> machine state in sync between two machines; you only need to quickly
> send diffs at critical moments. Then at any given moment, more than
> one host hardware can be hosting the identical internal system state
> for some guest VM.
>
> From vmware, the product they call "High Availability" just restarts
> the server on other hardware. (As far as I can tell from their tech
> docs.) Plus, Vmware doesn't do storage... So you'll have to get your
> own HA storage separately.
VMware has Fault Tolerance ("FT") which essentially keeps a mirror VM
running on another host. Literally a mirror. If you open the console
of them both simultaneously you'll see your mouse cursor move on both
at the same time. :)
This lets you survive a host outage in any case with some drawbacks.
>
> From virtualbox, you have live migration, but no HA. (As far as I
> can tell.)
>
> If you have a complex service, with apache, database, files, etc...
> The challenge is, what's the best way to make it HA? You can do some
> database replication, and use a load balancer, and either NFS or
> iscsi with mirroring, to make the filesystem HA. But that's a lot of
> stuff to possibly go wrong.
>
> The best I can find so far is to simply mirror the underlying storage
> of the guest OS, so in a failure, you can quickly easily boot the
> guest on another host. This is probably ok for a lot of purposes, a
> brief downtime in the event of failure. But I like to aim high
> whenever possible...
>
> Thanks for any comments/suggestions.
Best bet is still to make applications capable of surviving individual
VM failures, then distributing the application's VM's across multiple
environments IMO. Not always practical though. :)
Ray
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