Can zfs use mixed vdev mirror sizes

I have tested this with ZFS and write performance is about half what it should be, because ZFS distributes reads and writes over all vdevs (therefore dividing I/O to several places on the same disk). Thus, the speed is limited by the speed of the disk with the most partitions. Read speed seems to be equal to the disk bandwidth. Note a pair of ZFS partitions on two disks has roughly double the read speed of either single disk, because it can read from the disks in parallel.

Using MD LINEAR arrays or LVM to create the two halves results in twice the write performance compared to the above ZFS proposal, but has the disadvantage that LVM and MD have no idea where the data is stored. In the event of a disk failure or upgrade, one side of the array must be entirely destroyed and resynced/reslivered, followed by the other side. (e.g. the resync/resliver has to copy 2*(size of array))

Therefore it seems then that the optimal solution is to create a single ZFS mirror vdev across two LVM or MD LINEAR devices which combine the disks into equal-sized "halves". This has roughly twice the read bandwidth of any one disk, and write bandwidth is equal to the individual disk bandwidths.

Using BTRFS raid1 instead of ZFS also works, but has half the read bandwidth because ZFS distributes its reads to double the bandwidth, while it appears BTRFS does not (according to my tests). BTRFS has the advantage that partitions can be shrunk, while they cannot with ZFS (so if after a failure you have lots of empty space, with BTRFS it's possible to rebuild a smaller redundant array by shrinking the filesystem, then rearranging the disks).

This is tedious to do by hand but easy with some good scripts.

For two disks, you want mirror mode. This will cause the drives to be exact images of each other. If ZFS tries to read a block of data from the drive and it fails verification, then ZFS will automatically try to repair it. If the repair fails, it'll read it from the other drive and mark the bad drive as failed. You should get a notification that a drive failed, but the filesystem will still be usable. You get some read speed improvements, but no write speed improvements. As long as you have at least one drive from a mirror set, you have all of your data.

With raidz, you need at least three drives. They function similar to RAID5 where (effectively) one drive stores recovery information, and the other drives store data. It can handle any drive failing, as long as only one drive fails at a time.

raidz2 and raidz3 are the same as raidz, except that they can handle two or three drives failing, respectively. They require more drives to operate and reduce effective capacity, though.