Sequential write
Sequential mirroring performs writes to multiple copies or mirrors in order.
The multiple PPs representing the mirrored copies of a single LP are designated primary, secondary, and tertiary.
In sequential scheduling, the PPs are written to in sequence.
The system waits for the write operation for one PP to complete before starting the write operation for the next one.
When all write operations have been completed for all mirrors, the write operation is complete.
Parallel write
Parallel mirroring simultaneously starts the write operation for all the PPs in a LP.
When the write operation to the PP that takes the longest to complete finishes, the write operation is completed.
Sequential read
When a sequential read is specified, the primary copy of the read is always read first.
If that read operation is unsuccessful, the next copy is read.
During the read retry operation on the next copy, the failed primary copy is corrected by LVM with a hardware relocation.
This patches the bad block for future access.
Parallel read
On each read, the system checks whether the primary is busy. If it is not busy, the read is initiated on the primary.
If the primary is busy, the system checks the secondary and then the tertiary.
If those are also busy, the read is initiated in the copy with the least number of outstanding I/Os.