maintaining consistency by top posting. ;-)
All of what Curtis is saying matches what Sun says in its documentation
of fssnap snapshots and backing up with ufsdump. They have always said
that you should drop into single user mode to do backups. Nobody does,
because they just can't in a production environment. The alternative is
for your system to be as quiescent as possible. fssnap helps, because it
narrows the time window within which you have to be quiescent. You can,
if necessary, shut down certain programs briefly to take a snapshot.
But, even so, doing an fssnap and then ufsdumping the snapshot at, say,
2am in the morning, still has some risk that you will get an
inconsistent backup.
So, sysadmins routinely accept that small risk. Say it's a 1% risk. You
back up nightly with at least one full each week. The odds of not being
able to recover something reasonable then becomes much smaller than 1%.
There are too many other factors involved to say that these risks are
strictly multiplicative (1% times 1% yielding 0.01% risk of failure over
a 2 week span for example), but it does get smaller. As sysadmins we try
to understand these issues and make informed decisions that compromise
between the ideal and the performance requirements of our systems.
---------------
Chris Hoogendyk
-
O__ ---- Systems Administrator
c/ /'_ --- Biology & Geology Departments
(*) \(*) -- 140 Morrill Science Center
~~~~~~~~~~ - University of Massachusetts, Amherst
<[EMAIL PROTECTED]>
---------------
Erdös 4
Curtis Preston wrote:
Here's some further information from the VMware manuals. I searched for
the word snapshot in all manuals here:
http://pubs.vmware.com/vi35/wwhelp/wwhimpl/js/html/wwhelp.htm
First, you CAN take a snapshot of a powered-off machine. From the
manual:
"You can take a snapshot while a virtual machine is powered on, powered
off, or suspended. If you are suspending a virtual machine, wait until
the suspend operation has finished before taking a snapshot. You must
power off the virtual machine before taking a snapshot if the virtual
machine has multiple disks in different disk modes."
Also:
Note Snapshots of raw disks, RDM physical mode disks, and independent
disks are not supported.
Also from the manual:
"When you take a snapshot, be aware of other activity going on in the
virtual machine and the likely effect of reverting to that snapshot. In
general, it is best to take a snapshot when no applications in the
virtual machine are communicating with other computers. The potential
for problems is greatest if the virtual machine is communicating with
another computer, especially in a production environment.
For example, if you take a snapshot while the virtual machine is
downloading a file from a server on the network, the virtual machine
continues downloading the file, communicating its progress to the
server. If you revert to the snapshot, communications between the
virtual machine and the server are confused and the file transfer fails.
Another example is taking a snapshot while an application in the virtual
machine is sending a transaction to a database on a separate machine. If
you revert to that snapshot-especially if you revert after the
transaction starts but before it has been committed-the database is
likely to be confused."
This does not sound like I don't have to worry about the state of my
machine while taking a snapshot.
---
W. Curtis Preston
Backup Blog @ www.backupcentral.com
VP Data Protection, GlassHouse Technologies
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]
On Behalf Of Curtis Preston
Sent: Thursday, February 28, 2008 10:21 AM
To: Steven Kurylo; [email protected]
Subject: RE: Backing up VMware-VMs
After consulting with two VMware experts, I stand by my original
statement, that a snapshot of a vm is a crash consistent copy, which
means that it's just like someone pulled the power plug. It is NOT like
someone suspending a VM.
There are 3rd party products that provide additional levels of
consistency (netapp, falconstor that I know of), but the default VMware
snapshots are crash consistent, not transactional consistent.
---
W. Curtis Preston
Backup Blog @ www.backupcentral.com
VP Data Protection, GlassHouse Technologies
-----Original Message-----
From: Steven Kurylo [mailto:[EMAIL PROTECTED]
Sent: Wednesday, February 27, 2008 2:06 PM
To: Curtis Preston; [email protected]
Subject: Re: Backing up VMware-VMs
Curtis Preston wrote:
Unless you're coordinating with the OS, then taking a VMware snapshot
and copying it is equivalent to pulling the power plug on a server.
Will it power back up without corruption? 99.9% of the time, yes. Has
anyone who has been in the biz for a while had a scenario where
powercycling a box caused a corrupted OS disk? I'd say so.
Thats false.
Its the same thing as suspending a VM. When you restore the you can
either restore it as if the power was shut off or resume it as if
nothing had happened. Obviously the outside world has changed, so
network connections, the time, etc, will have changed.
