[dm-devel] Device mapper for temporarily buffering writes on fast device before pushing to slower device

[Heinz Mauelshagen]

dm-writecache (see list archieve) is what you're looking for.


On 02/19/2018 09:47 PM, Drew Hastings wrote:
> I found stochastic multi-queue and writeboost to be insufficient for 
> this purpose. I'm wondering if anything exists that fits this 
> description:
>
> Device mapper creates a "cache" device on a fast device (SSD/NVME, 
> etc)... and writes to the device *always* hit the fast device. Writes 
> are later flushed to the slower device as fast as the slow device can 
> handle them.
>
> The problem with the caching solutions I have found with device mapper 
> is that they require the block to be identified as "hot" first, even 
> when the cache itself is not 100% utilized. Read performance is 
> largely irrelevant.
>
> Is there a device mapper implementation that works in this fashion; 
> where 100% of writes happen to the fast device? I was thinking COW 
> effectively does this, but the problem is it does not 
> automatically/periodically flush writes, you have to do them all at once.
>
> I had seen some conversations about trying to get the multiqueue (mq, 
> not smq) policy to do this with zero values for thresholds, but it did 
> not seem like any were successful in ensuring the first write would 
> hit the fast device.
>
> Thanks!
>
>
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