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I have read that COW filesystems tend to perform terribly on HDD with database workloads because overwrites mid-file are common and increase external fragmentat

• johannes1234321 10y ago
  The answer to that is: use an SSD. With SSD random seek vs sequential access doesn't matter. Second part of the answer is that ZFS provides features for cache devices. So you get a fast disk as cache and a set of larger disks for as actual storage. All writes at first go to the cache device to be safe and then are written to actual storage, thanks to cow this can be done in larger sequential sequences. (Unless the disks become too full, ZFS has issues when disks are ~80% full, then it takes too much time to find free space) What's left are longer "sequential" read operations. There ZFS tries to predict the read, this can be successful, but might also happen that the seeks take too much time for highest performance.