Re: A target for hotswap?

[Mike Snitzer <snitzer@redhat.com>]

On Mon, May 04 2015 at  6:37am -0400,
Akira Hayakawa <ruby.wktk@gmail.com> wrote:

> Hi,
> 
> Some DM target requires additional device that could be a single point of failure.
> Let's consider dm-cache.
> dm-cache requires a HDD and a SSD and if SSD is dead, the data is lost.
> To protect from this, SSD can be built RAID. But RAID isn't an ideal solution with SSD
> because the devices can be dead at the same time. Cost is the another problem of course.
> 
> So, let's think about hotswap.
> A hotswap device has an active device and multiple ready devices.
> The active device is usually used and when the device is ordered to swap by
> a ready device it starts to sync the two devices and change the active device
> when sync is over. It's not simply dd-ing the devices because I/O isn't suspended and
> not all region is necessarily copied.
> You can order swapping whenever you like to. For example, swapping every week can be
> double the longevity of the cache device.
> 
> I don't know how I can do this with DM targets.
> Do other part of linux technologies have such functinality? Please teach me if so.

You could probably use lvm2's pvmove (it uses old dm-mirror target to swap over).