FR1 is a Linux kernel driver that adapts the kernel's own software RAID1 driver.
It's intelligent in that it doesn't blindly resynchronize a whole mirror component when only a few blocks need resyncing. That can save hours of resync time on a large device. In addition, it always chooses the fastest device to read from on reads.
The driver keeps a bitmap of pending writes in memory, and writes them to the mirror component that's just been repaired when it comes back on line. The bitmap is two-level and created pagewise on demand, so it's not too expensive.
A terabyte sized device with blocks of 4K will cost max 32MB of memory per mirror component, thus 64MB max for a two component mirror.
But in practice only a few pages of bitmap will have been allocated by the time you come to fix the device.
Limitations:
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