Scheduler Layer

The scheduler layer schedules physical requests for logical operations and handles mirroring and the MWC cache. For each logical request the scheduler layer schedules one or more physical requests. These requests involve translating logical addresses to physical addresses, handling mirroring, and calling the LVDD physical layer with a list of physical requests.

When a physical I/O operation is complete for one phase or mirror of a logical request, the scheduler initiates the next phase (if there is one). If no more I/O operations are required for the request, the scheduler calls the strategy termination routine. This routine notifies the originator that the request has been completed.

The scheduler also handles the MWC cache for the volume group. If a logical volume is using mirror write consistency, then requests for this logical volume are held within the scheduling layer until the MWC cache blocks can be updated on the target physical volumes. When the MWC cache blocks have been updated, the request proceeds with the physical data write operations.

When MWC is being used, system performance can be adversely affected. This is caused by the overhead of logging or journalling that a write request is active in one or more logical track groups (LTGs) (128K, 256K, 512K or 1024K). This overhead is for mirrored writes only. It is necessary to guarantee data consistency between mirrors particularly if the system crashes before the write to all mirrors has been completed.

Mirror write consistency can be turned off for an entire logical volume. It can also be inhibited on a request basis by turning on the NO_MWC flag as defined in the /usr/include/sys/lvdd.h file.