Dominic Coulombe wrote:
On 7/24/06, David Boyes <[EMAIL PROTECTED]> wrote:
One more time: Unless your Linux systems *are completely down* at the
time of backup, full volume dumps from outside the Linux system are more
than likely to be useless.
Can you explain why is that ?
To avoid the nitpickers, let's say that David means all filesystems must
be flushed and ro.
As I understand it, journalling (by default) logs metadata (dirctory
info) but not data.
If you create a file, that's journalled. If you extend a file, that's
journalled. The data you write to the file are not.
Let's say that you create a file, write 4K to it, close it. Let's say
you do a backup of the volume externally while the 4K data remains
unwritten. Note: read in "man 2 close" "A successful close does not
guarantee that the data has been successfully saved to disk."
So now you have journalled (or comitted) metadata that says the file's
got 4K of data in it.
But, it hasn't. In the ordinary course of events, the data gets written
to disk ans all is well.
The same sort of thing happens when a file's updated in place, as I
expect databases commonly are.
If a database product says its backup program works with active
databases, I expect it does, but I'd never trust an external program,
let alone an external system, to backup my database, unless the database
is down.
I never experimented such failure after doing live backup of journaled
filesystems.
Have you looked for a failure?
I think it more likely you've had a failure that you didn't notice than
that you didn't have a failure.
I've never noticed a problem with losing data due to a power failure
(except when it took the hardware with it!), but I'm not so foolish as
to assume that I've had no file corruption.
It is like brute forcing a shutdown by logging off the VM machine : not
ideal, but not supposed to break your Linux machine. It is the reason to
use journaled filesystems.
Thanks.
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Cheers
John
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