====== 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: [[https://www.redhat.com/en/blog/improving-read-performance-dm-cache|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
  * [[https://wiki.archlinux.org/index.php/bcache|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:
  * [[http://strugglers.net/~andy/blog/2017/07/19/bcache-and-lvmcache/|bcache and lvmcache]]


===== flashcache =====

<code bash>
apt-get install flashcache-dkms flashcache-utils
</code>

Reference [[https://wiki.archlinux.org/index.php/flashcache]]