The previous resilvering algorithm repairs blocks from oldest to newest, which can degrade into a lot of small random I/O. The new resilvering algorithm uses a two-step process to sort and resilver blocks in LBA order.
The amount of improvement depends on how pool data is laid out. For example, sequentially written data on a mirrored pool shows no improvement, but randomly written data or sequentially written data on RAID-Z improves significantly - typically reducing time by 25 to 50 percent.
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