In this lesson, you will learn how to choose between volatile and persistent write-caching modes in LVM to balance speed and data safety.
When you use LVM cache tiering, you are essentially placing a fast, small device (like an SSD) in front of a slower, large device (like a mechanical HDD). The "write-caching strategy" dictates how the system handles the data moving between these two layers.
Write-Back (Volatile Performance)
In write-back mode, the LVM cache acknowledges a write operation as "complete" as soon as it hits the fast SSD. It then slowly synchronizes that data to the slower main disk later.
- Pros: Incredible performance, as the system doesn’t wait for the slow HDD.
- Cons: If power is lost before the data moves from the SSD to the HDD, that data is permanently lost. This is considered volatile because the cache itself becomes a single point of failure.
Write-Through (Persistent Durability)
In write-through mode, the system does not acknowledge a write as complete until it has been successfully written to both the SSD cache and the main storage disk simultaneously.
- Pros: High durability. If you lose power, your data is already safe on the main disk.
- Cons: Write performance is limited by the speed of your slowest disk (the HDD). It is persistent because the cache acts only as a read-accelerator; writes are always committed to the underlying durable storage.
Choosing Your Strategy
You configure these modes when attaching the cache pool to your logical volume using the --cachemode flag. For example:
# Setting up for maximum performance
lvconvert --type cache --cachemode writeback vg00/my_lv /dev/ssd_cache
# Setting up for maximum data safety
lvconvert --type cache --cachemode writethrough vg00/my_lv /dev/ssd_cache
If your application requires high-speed logging or temporary buffers, writeback is usually preferred. If your application handles critical database transactions or system binaries that must not be corrupted by a power flicker, writethrough is the safer choice.
Key Takeaway: Use writeback for maximum performance when data loss risk is acceptable, and writethrough for maximum durability where data integrity is the priority.