Storage devices have different characteristics in terms of performance, capacity, and cost. For example, solid state drives (SSDs) are faster but more expensive than hard disk drives (HDDs). To optimize the use of storage resources, it is desirable to combine different types of devices in a way that balances speed and space. This is where multi-tiered storage strategies come in.
Multi-tiered storage is a technique that uses multiple layers of storage devices with different performance and capacity levels. The idea is to store frequently accessed or critical data on the faster devices, and less frequently accessed or less important data on the slower devices. This way, the overall performance and efficiency of the storage system can be improved.
There are two main approaches to implement multi-tiered storage on Linux: LVM and Bcache. LVM stands for Logical Volume Manager, a tool that allows creating, resizing, and managing logical volumes from physical volumes. Bcache stands for Block Cache, a kernel module that allows using an SSD as a cache for a HDD. Both LVM and Bcache can be used to create a hybrid storage system that combines the advantages of SSDs and HDDs.
LVM has a feature called LVM cache, which allows creating a cache pool from an SSD and attaching it to a logical volume on a HDD. The cache pool consists of two logical volumes: one for cache data and one for cache metadata. The cache data volume stores the actual data blocks that are cached, while the cache metadata volume stores the information about which blocks are cached and where they are located. LVM cache uses the dm-cache kernel module to manage the cache pool and the logical volume.
Bcache has a similar concept of cache and backing devices. The cache device is an SSD that stores the cached data blocks, while the backing device is a HDD that stores the original data blocks. Bcache uses the bcache kernel module to manage the cache and backing devices. Bcache also supports multiple cache devices and multiple backing devices, as well as different cache modes and policies.
Both LVM cache and Bcache have their pros and cons. LVM cache is more integrated with the LVM toolset and can be easily configured and managed using the LVM commands. LVM cache also supports encryption and snapshots for the logical volumes. However, LVM cache has some limitations, such as the requirement to use the same sector size for the cache pool and the logical volume, and the inability to resize the cache pool or the logical volume without detaching them first.
Bcache is more flexible and powerful than LVM cache, as it supports more features and options, such as different cache modes (writeback, writethrough, writearound), different cache policies (lru, fifo, random), and different cache set features (sequential cutoff, writeback delay, etc.). Bcache also allows resizing the cache and backing devices without detaching them, and supports encryption and compression for the backing devices. However, Bcache is more complex and less user-friendly than LVM cache, as it requires more steps and commands to set up and manage, and has less documentation and support.
In conclusion, LVM and Bcache are two viable solutions for implementing multi-tiered storage on Linux. They both offer the benefits of combining SSDs and HDDs in a hybrid storage system, but they also have their trade-offs in terms of ease of use, functionality, and compatibility. Depending on the specific needs and preferences of the user, either LVM or Bcache can be a suitable choice for leveraging multi-tiered storage on Linux.