Lenny wants to create a striped RAID-0 with his 2TB drives, and then he wants to mirror that whole array to a single 8TB hard drive. He has 4x 2TB SATA hard drives and 1x 8TB hard drive. Because of the -write-mostly option preceding his "slow" devices, his computer will perform all write operations on both drives, but most of his read operations will come from /dev/sda. In this example, /dev/md0 will be the new RAID-1 volume, /dev/sda is Lenny's internal SATA drive, and /dev/sdb is his USB-connected external drive. $ sudo mdadm -create -verbose -assume-clean /dev/md0 -level=1 -raid-devices=2 /dev/sda -write-mostly /dev/sdb Lenny can use the following command to configure the USB drive as a "write-mostly" drive so that when the computer boots and he does his computery stuff, most of the reads come from his internal drive:
(Lenny is aware that RAID'ing a USB device is a bad idea, but he's got what he's got, and his fancy RTX 2080ti wasn't cheap, so another drive isn't in the budget right now.)
Lenny wants to configure the 2 drives in a RAID-1 mirror so that if either of the 2 drives fails, his computer will remain functional until the failed hardware can be replaced. Lenny's computer has a single 2TB SATA hard drive, and he has an external 2TB hard drive connected via USB. Meteor strike would be a different story, but he would prefer to keep his computer with maximum up-time. He has an off-site backup solution in place, but he really doesn't have time time/resources to recover from backup for something as simple as a failed hard drive. These would be time-consuming to re-download in the case of a drive failure. He also has lots of movies, music, and other files that take up lots of space. is building a computer that has lots and lots of video game ROMs and disk images. However, this option does allow for some interesting and creative RAID solutions, especially when mixing storage media of different sizes/speeds. This option is most useful when creating a RAID-1 mirrored array where the participating volumes might be connected over a variety of media (i.e., network/USB) in combination with faster connections (SAS/SATA/fiber). This option is used to mark a participating volume as a "slow-performing" volume so that most of the reads from an array will come from other, faster, volumes.
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