Mirror write cache and consistency

[brian_31]

Can someone explain these two parameters for turning them off and advantages and when we would apply them?

Thanks
Brian

[Craig Rants]

I seem to remeber that an HP Engineer once recommended turning Mirror Write Cache off on swap, something to do with crash dumps and them getting overwriten.

[James R. Ferguson]

Hi Brian:

An excellent document discussing this is #KBAN00000180 from the Technical Knowledge Base (attached). You can also find some information in the man pages for 'lvcreate' and 'lvchange'.

Regards!

...JRF...

[brian_31]

Hi Craig:

Do you know why? and how to turn them off. Are they *on* by default.

Thanks
Brian

[Uday_S_Ankolekar · Accepted solution]

Hi,

lvchange command can take care of this...
man page says..
-c mirror_consistency Set mirror consistency recovery. This option is effective only when -M n is specified or previously set mirror_consistency can have one of the
following values:
y Set mirror consistency recovery on.
LVM achieves mirror consistency
during volume group activation by
going through all logical extents
and copying data from a nonstale
copy to other mirror copies

n Set mirror consistency recovery off. LVM does not perform mirror consistency recovery on this logical volume when the volume group is activated

[James R. Ferguson]

Hi (again) Brian:

With regard to your swap-related question:

Knowledge Base document #KBRC00008230 notes, "It is optional to turn off mirror write cache and mirror consistency recovery
for primary swap when mirrored. The drawback to leaving these options on is the added overhead for the mechanisms that insure a mirror is right. Other than the added overhead, leaving these options on should not cause any problems with the functionality of primary swap or primary swap when it is used as a dump device."

Regards!

...JRF...

[S.K. Chan]

This is what I understand ..
Mirror Write Cache is a way to make sure that a "write" is ONLY committed if all mirrored copies are successfully made.
Mirror Consistency Recovery, is a mechanism by which a mirror is checked when an unexpected shutdown is performed.

It is optional to turn off MWC & MCR for primary swap when mirrored. The only disadvantage of leaving them ON is that you have to live with the aditional overhead for making sure a mirror is right.
Other than that leaving them ON is not a problem.