[JDSS] Write log usage and redundancy

Additional information:

• product name: JovianDSS
• product version: all versions 
• build: all versions

Subject:

Recommended usage and disk redundancy for write log configuration.

Contents:

1. Write log, also called as ZIL or write cache is a place where short term data is stored. It provides a great impact on the performance of any environment, by increasing the speed of write operations.

2. When creating a ZIL there are few important aspects that need to be addressed. First of all, ZIL is always created per pool. Preferably, it should consist of disks with higher IOPS than the ones that create a pool, for example, if the pool consists of SSD discs the write log should be created with SSD NVMe disks, to obtain maximum performance impact for the environment.

3. Redundancy of disks that form ZIL is crucial, as the data is passing through the ZIL before it appears on the pool storage, therefore in case of using only one disk as ZIL in case of failure data loose will occur.