Re: Simultaneous assembly on multiple hosts

[Neil Matai <neil@matai.kiwi.nz>]

On Fri, May 03, 2013 at 08:25:59PM -0600, Marcus Sorensen wrote:
> > Is it dangerous have an array assembled on two hosts at the same time?
> > Say I have servers A and B exporting devices a and b via ATA over
> > Ethernet or similar to hosts C and D.  If a and b are components of an
> > array then both C and D automatically assemble the array as soon as
> > the components a and b are exported but is this safe (on the proviso
> > that only one of C or D is ever writing to the array at any time and
> > that there is no write-behind caching above the array device)?
> Yes, it is dangerous. There are some limited situations where it may work,
> without a bitmap and some cluster aware application in control of what
> blocks are written, but you can't for example mount it ext4 and write to
> only one side at a time. That's why people use drbd.

Sorry.  I should have said what it was for.  I realise that mounting a
typical file system twice would be dangerous (also with drdb).  The
intention would have been to live migrate guest systems between C and
D so the file system remains mounted (once) inside the guest system
but nowhere else.  Would it work in this scenario without a bitmap or
would this scenario also lead to data loss?  Would it make any
difference if 0.9 metadata were use to create the array?

Many thanks,

- neil