From mboxrd@z Thu Jan  1 00:00:00 1970
From: Akira Hayakawa <ruby.wktk@gmail.com>
Subject: Re: A target for hotswap?
Date: Wed, 06 May 2015 10:34:32 +0900
Message-ID: <55496FA8.3090504@gmail.com>
References: <55474BEE.1090402@gmail.com> <20150504160639.GA1493@redhat.com>
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To: Mike Snitzer <snitzer@redhat.com>
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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).
>