Linux – LVM cache and TRIM

You can do this fairly simply. Kinda surprised there wasn't an answer for this here already.

You can do this entire process while running on the filesystem you want to resize (yes, it's safe and fully supported). There is no need for rescue CDs or alternate operating systems.

1. Resize the partition (again, you can do this with the system running). GParted is easy to use and supports resizing.
   You can also use a lower level tool such as fdisk. But you'll have to delete the partition and recreate it. Just make sure when doing so that the new partition starts at the exact same location.
2. Reboot. Since the partition table was modified on the running system, it won't take effect until a reboot.
3. Run pvresize /dev/sdXY to have LVM pick up the new space.
4. Resize the logical volume with lvextend. If you want to use the whole thing, lvextend -r -l +100%FREE /dev/VGNAME/LVNAME. The -r will resize the filesystem as well.
   Though I always recommend against using the entire volume group. You never know what you'll need in the future. You can always expand later, you can't shrink.

To my best understanding, dm-cache does what you are asking for. I could not find a definite source for this, but here the author explains that he should have called it dm-hotspot, because it tries to find "hot spots", i.e. areas of high activity and only caches those.

In the output of dmsetup status you will find two variables, namely read_promote_adjustment and write_promote_adjustment. The cache-policies file explains that

Internally the mq policy determines a promotion threshold. If the hit count of a block not in the cache goes above this threshold it gets promoted to the cache.

So by adjusting read_promote_adjustment and write_promote_adjustment you can determine what exactly you mean by frequently read/written data and once the number of reads/writes exceed this threshold, the block will be "promoted" to, that is, stored in, the cache.

Remember that this (pre-cache) metadata is usually kept in memory and only written to disk/SSD when the cache device is suspended.