HDD on SSD caching

Speed up HDDs with additional SSD/NVMe caching.

Options:

OpenZFS
- it is possible to add cache and log devices to ZFS pool. Log is a journal for synchronous writes.
LVM DM-Cache
- No HDD reformating if it is already using LVM
- it is possible to add cache device to logical volume: Improving read performance with dm-cache
- https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/logical_volume_manager_administration/lvm_cache_volume_creation
- It is hot-spot cache, to cache only frequently accessed data, so simple copy of big file doesn't fill cache.
- Write cache is also possible: https://man.archlinux.org/man/lvmcache.7
bcache
- need to reformat HDD

Writeback caching:

Filesystem corruption when SSD cache fails. Possible workaround for mirroring FS is to use 2 separate caching devices for 2 mechanical drives.

Benchmarks:

flashcache

apt-get install flashcache-dkms flashcache-utils