[dm-devel] [RFC][PATCH 0/4] dm-log: support multi-log devices

[malahal at us.ibm.com]

Takahiro Yasui [tyasui at redhat.com] wrote:
> > Not really. What I meant is re-integrating a failed log device when it
> > comes back again. That is also what I mean by handling 'transient
> > errors'.
> 
> Thanks, I see your requirement on this feature.
> Let me put one more question.
> 
> LVM mirroring is used to make system available even if some devices,
> such as data and log device, have a problem. Currently, activations
> by "vgchange -ay" command seem to fail during system booting if one
> of devices related to VG are missing or had I/O error. For example,
> if a mirror is structured by two data devices and log devices, the
> mirror logical device should be activated and used even if one data
> device and one log device are missing.
>
> Could you give me an idea or solution to handle this? Do we need to
> enhance your feature to achieve this requirement?

The requirement you are asking is more than "mirrored log" support.
Actually it is nothing to do with "mirrored log" support! LVM has
"--partial' option but that creates read-only volumes.  IBM has
implemented "--partial-rw" that activates such mirror devices.
Essentially it creates an 'error device' in place of a missing device,
but the patch isn't complete as it doesn't work well with other segment
(target) types!

What we need is changes to LVM "--partial" code path where it can create
volumes read-only or read-write based on what is available instead of
blindly doing read-only.

Thank you for looking at these enterprise level features.

Thanks, Malahal.