On 14/08/2014 07:56, NeilBrown wrote:
....
As --freeze-reshape is only meaningful in combination with --assemble,
this question doesn't really make sense.
If you are using a sufficiently new kernel and mdadm so that "data_offset" is
adjusted during reshapes so that no 'backup' is needed, then you can
suspend a reshape for a period of time by:
echo frozen > /sys/block/mdXXX/md/sync_action
This is perfectly safe. When you want to unfreeze, write 'idle'
to 'sync_action'. md will notice that a reshape is pending and will restart
where it was up to.
.....
Is it really that esoteric?
If you want to reshape an array, you run "mdadm --grow" and list all the
changes you want to make. Set a new level, a new number of devices, a new
layout, a new chunk size, whatever. mdadm will do it if it can and give an
error if it cannot.
If you want to test it out first then that is extremely sensible. Make some
loop devices and experiment.
I also was interested in the functioning of reshape, and it never was
completely clear to me.
Do you confirm that the manpage for --freeze-reshape is correct?
--freeze-reshape
Option is intended to be used in start-up scripts
during initrd boot phase. When array under reshape is assembled during
initrd phase, this option stops reshape after
reshape critical section is being restored. This happens
before file system pivot operation and avoids loss of file system
context. Losing file system context would cause
reshape to be broken.
"would cause reshape to be broken" means complete data loss?
So, to be safe in case of power loss during reshape, we have to make
absolutely sure that our Linux distribution implements the
--freeze-reshape before pivot_root ?
I think Ubuntu doesn't do that. Debian probably also doesn't.
Is the reshape operation supposed to be safe with regard to power loss
(supposing the distro implements --freeze-reshape) ?
Is the --freeze-reshape needed only when there is a backup file, or even
with newer kernels (>= 3.5) and mdadm (>= 3.3) and no backup file ?
If mdadm lets me initiate a reshape without specifying a backup file,
does it mean that it has checked that in my case it is safe?
Thank you
EW
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
