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Am I correct that optimal chunk size is average file read/written to disk divided by number of block devices in RAID array storing the data? For example if the average file size is 1024KiB and I have four disks in RAID1, then I should choose the chunk size around 256KiB? Or if I have two drives in RAID0, then I should choose the chunk size 512KiB instead? Or are there better methods/benchmarks to determine the optimal chunk size for software-RAID? Last but not least, is there a good utility which could help one to measure the average I/O read/write size?

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Take a look at this wiki page. It explains chunk size in pretty good detail. Also take a look at this page. It has benchmarks galore including links to benchmarking utilities. – yoonix Mar 6 '14 at 3:54

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