Factors Influencing Optimal Dd Block Size For Cloning Disks And Partitions
What Influences dd Block Size Performance?
When using the dd command to clone disks or partitions in Linux, the block size parameter (-bs) specifies the size of the blocks of data that are copied at a time. The optimal block size can vary widely depending on several factors related to the source and destination storage devices and file systems.
Drive Speeds Limiting Throughput
One major factor is the read and write speeds of the source and destination drives. If using a fast SSD as the source, and a slower mechanical hard drive as the destination, the maximum write speed of the hard drive will bottleneck the cloning operation before the SSD’s maximum read speed is reached. There is no benefit to setting the block size higher than the destination’s write speed can support.
Partition Alignment Causing Slowdowns
Another factor is partition alignment between the source and destination partitions. If the partitions are not aligned to cylinder or stripe unit boundaries on their respective devices, read and write speeds will be much slower due to excessive drive head movement or split stripe writes. Aligning the partitions first can allow higher performance at a given block size.
Filesystem Block Sizes Mismatched
If the source and destination partitions have different filesystem block sizes, such as 4KB for Ext4 and 64KB for XFS, there can be read-modify-write amplification if the dd block size does not cleanly match up with both. Ideally the dd bs should be a multiple of both filesystem block sizes, or at minimum match the larger of the two.
Benchmarking to Find Sweet Spot
Given all the potential variables, the most accurate way to optimize dd block size for a specific cloning task is to benchmark various block size values and measure the overall throughput.
Example dd Benchmarking Code Snippet
dd if=/dev/sda of=/dev/null bs=8k count=100000 125358983 bytes (125 MB, 119 MiB) copied, 1.06427 s, 118 MB/s dd if=/dev/sda of=/dev/null bs=128k count=10000 1363148800 bytes (1.4 GB, 1.3 GiB) copied, 9.15184 s, 149 MB/s
By benchmarking with different bs values, the value that provides the fastest throughput can be identified. Using a null output sink with count limits io to measure the source read speed isolated from destination write speed.
When Smaller is Faster: Random I/O Workloads
For cloning partitions with lots of small, random access files like system partitions with many small configuration and log files spread throughout, smaller block sizes almost always provide better throughput. The reads on source and writes to destination in this case are small and random, not sequential. So the overall operation stays more efficient reducing excess data copied and write amplification.
When Bigger is Better: Linear Large Copies
On the other hand, when cloning large files with streaming sequential I/O like VM disk image files or media files, its best to maximize the block size to take advantage of the drives’ caching and read ahead capabilities. The sequential transfers will achieve maximum throughput with larger block sizes up to the maximum the destination drive can support per operation.
Recommendations for Common Use Cases
Cloning OS Partitions: 4-256KB Blocks
When cloning smaller partitions like /boot, /root or /home partitions containing operating systems, set the dd bs between 4KB and 256KB for best results. Match the filesystem’s native block size if possible. Smaller random transfers will be most efficient.
Cloning VM Disk Images: 1-8MB Blocks
For larger sequential files like VM disk images on SAN storage or SSDs, start testing dd bs values between 1MB to 8MB. Monitor overall throughput and increase until maximum sustainable write speed of destination is reached.
Overlay File Cloning: 128KB-1MB Blocks
Overlay-based container images and VM disks with mixed random and sequential I/O may benefit from intermediate block sizes between 128KB to 1MB. Match the backing filesystem block size if possible as the baseline.
