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xen-users
Re: [Xen-users] Xen with HA
Tim Post wrote:
On Mon, 2007-01-08 at 12:18 -0200, Marco Sinhoreli wrote:
Hello everybody,
We are thinking about using virtual machines for a scene of high
availability where is contemplated the division of the VMs in two hosts.
Considering that both hosts will have 6 VMs and in the host A will be
active the VMs vm1, vm3 and vm5 , while in the host B will be active
the VMs vm2, vm4 and vm6. For each VBD allocated in one LV (Logical
volume of the LVM) we pretend to implement the following architecture.
P = primary | S = Secondary
------------------------------------
Host A
------------------------------------
vm1 vm2 vm3 vm4 vm5 vm6
| | | | | |
drbd1 drbd2 drbd3 drbd4 drbd5 drbd6
P S P S P S
------------------------------------
| | | | | |
| | NETWORK | |
| | | | | |
------------------------------------
S P S P S P
drbd1 drbd2 drbd3 drbd4 drbd5 drbd6
| | | | | |
vm1 vm2 vm3 vm4 vm5 vm6
------------------------------------
Host B
------------------------------------
Consider that, where the block device is primary, the VM is active on
the host, and where the device blocks is secondary, the VM is inactive
on this host.
You may find this is easier utilizing network attached (centralized)
storage in lieu of block level mirroring, however your explanation is
somewhat confusing.
>From your block diagram, it looks as though you plan to live-migrate in
the event of failure or scheduled shutdown, using nbd to maintain the
file system for each of the 4 components making up host A or B, each
having separate file systems. Is this correct?
No, not is correct. In etch of the 6 componets (vm{1-6}) I have one drbd
connection for mirroring between 2 Logicals Volume LVM. Example: vm1 use
the drbd1 that is attached in /dev/vg/vm1-disk in each hosts. The LV
/dev/vg/vm1-disk exist in each host and has the same size. The drbd
block is active only in host A or B. Supposed that the vm1 is actived at
host A. If host A was crashed, the heartbeat at host B detect it and
active the drbd1 and boot the vm1.
Other possibility is scheduler shutdown, in this case the host A stop
the services and the heartbeat service do a live-migration of the vm1
and desactive the drbd1. At host B, the heartbeat service receive the
call for active the resource vm1 and active the drbd1 and verifies if
vm1 is active, in afirmative case, finishes the script, in negative
case, call the boot of the vm1.
Other alternative is to use drbd8. It allows the drbd work with block
devices active in 2 hosts as in the NAS. But drbd8 is an alfa version.
The heartbeat will have one responsible script for the VMs allocation in
case of hardware failure from one of the hosts for your even. In case of
programmed stop, the host from the VMs will do the migration for it's
even without the stop of the services that are been executed. Will occur
a prolongated downtime and full lost of the VMs activities only in case
of a abrupt stop of the servers hardware.
So at no point will two active VM's be accessing the same file system,
correct? This would be easier if the file systems were on a raid backed
NAS.
The vm1 at Host A and B not access the same file system at the same
time. Only one host the vm1 will access the file system as I described.
We wish information from the list if this solution have been done by
someone,in positive case, if there is some documentation that
contemplate this implementation ,and in negative case, how to do an
implementation of a HA integrated with Xen.
Your solution should work exactly as you described it, if I'm
understanding it correctly. My only recommendation is to ditch block
level mirroring and go with centralized storage.
ok
You can, of course, use NBD to mirror the storage if a decent RAID
doesn't give you the desired comfort level. What you have , in essence
is four single pole double throw switches (an over simplified example),
ensuring there is no "short" (no two P/S domains access the same FS at
once) and you're fine :)
A little over paranoid considering I/O costs, but should work well :)
You have done a similar configuration of HA and Xen using DRBD and
heartbeat?
Regards,
Best regards
Marco Sinhoreli
Linux expecialist
Samurai Projetos Especiais
São Paulo - Brazil
Best,
--Tim
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