Re: [linux-lvm] Powerfailure and snapshot consistency

[Ray Morris <support@bettercgi.com>]

On Sat, 26 Mar 2011 00:24:56 -0400 (EDT)
"Stuart D. Gathman" <stuart@bmsi.com> wrote:

> Assuming my theory is correct, the morals would be:
...
> 2) make sure important LVs do not span multiple PVs (except for LVM
>     mirroring) - you could be unhappy in the event of a system crash.

   I don't believe that multiple PVs necesarily makes a 
difference. On a single PV, there is no guarantee that 
writes make it to disk during a power failure, what order
they make it in, or even if a write is correct while voltage
is dropping. The drive itself could reorder a write (NCQ), 
the raid card could do so, etc. One might say the moral
of the story is "don't run important production servers 
on spotty power systems without an effective UPS".  

  Someone mentioned journaling. Journaling does not of
course guarantee data consistency, only makes meta data, 
more likely to be consistent, so regardless when you 
undervolt a drive during power failure something is subject
to go wrong, aside from the more obvious effects of even
an instant power cut while writing a block.  Moral - 
don't yank the power is the data is important.
-- 
Ray Morris
support@bettercgi.com

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