A Comparison of Chunk Size for Software RAID-5

Linux Software RAID Performance Comparisons

The Problem

Many claims are made about the chunk size parameter for mdadm (--chunk). One might think that this is the minimum I/O size across which parity can be computed. There is poor documentation indicating if a chunk is per drive or per stripe. This study determines that.

The Controller

The Test System

The Test Matrix


Different chuck sizes have different performance characteristics for different I/O sizes and workload types. In general 128KB provides the most consistent and highest throughput, except when doing random reads using an I/O that is a multiple of the stripe sizes (e.g., 256KB I/Os on a 4+1 RAID-5 with 64KB chucks). For 3+1 and 4+1 RAID-5s, I recommend a chuck size of 128KB for the best overall throughput characteristics.

Sequential Single Threaded Tests

For 100% sequential reads, we see that a chunksize of 1024 has maximum throughput for 4MB I/O sizes. This is compatible with the strip size being the number of data disks multiplied by the chunk size. Unfortunately, no datapoints were taken for a 3-spindle RAID-5 at I/O sizes of 3 times the chunk size. Clearly, chunk sizes that are too large or too small have pathological behavior at certain I/O sizes. Chuck sizes of 64KB and 128KB have good performance through 1MB I/O sizes without notable pathology.

Random Single Threaded Tests

Again, chuck sizes of 64KB and 128KB have reasonable performance across I/O sizes. 64KB may be better for 256KB writes.

Sequential Multi-Threaded Tests

Here the 64KB chunksize does significantly worse than 128KB for 100% random reads.

Random Multi-Threaded Tests

For multi-threaded 100% random writes, smaller chuck sizes seem better, with 32KB providing peak performance. Again, 64KB and 128KB chuck sizes are consistent across I/O sizes, except for 100% random reads, where 1MB chucks provide consistent peak performance.