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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