A target for hotswap?

A target for hotswap?

[Akira Hayakawa]

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.

- Akira

Re: A target for hotswap?

[Mike Snitzer]

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).

Re: A target for hotswap?

[Akira Hayakawa]

Thanks Mike,

Great.

I just look this picture and start to think pvmove must be what I want.
http://www.redhat.com/magazine/007may05/features/storage/figs/pvmove.jpg

- Akira

On 2015/05/05 1:06, Mike Snitzer wrote:
> 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).
>