Why Windows Server 2012 R2 Parity Storage Spaces Perform Slowly?
The caveat of a parity space is low write performance compared to that of a simple or mirrored storage space, since existing data and parity information must be read and processed before a new write can occur. Parity spaces are an excellent choice for workloads that are almost exclusively read-based, highly sequential, and require resiliency, or workloads that write data in large sequential append blocks (such as bulk backups).
What is Journal?
The sequential write performance of parity spaces can be improved by using dedicated journals. A journal is used to stage and coalesce writes to provide resiliency for in-flight I/Os. This journal resides on the same disks as the parity space unless you designate physical disks in the pool as journal disks. The journal is a mirror space and thus resilient by itself. The advantage of dedicated journal disks is a significant improvement in sequential write throughput for parity spaces. Incoming writes are de-staged to the parity space from dedicated disks, thus significantly reducing seeking on the disks used by the parity space. Using THREE SSDs as a dedicated journal to a three disk parity space, a sequential throughput increase of ~150% was observed under lab conditions. Note that the throughput of the journal disks will now be the overall throughput limit to all parity spaces created on this specific storage pool and you might trade extra capacity for performance. In other words, ensure that dedicated journal disks are very fast and scale the number of journal disks with the number of parity spaces on the pool.
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