Disk Performance

The Solaris DDI/DKI provides facilities to optimize I/O transfers for improved file system performance. A mechanism manages the list of I/O requests so as to optimize disk access for a file system. See Asynchronous Data Transfers (Block Drivers) for a description of enqueuing an I/O request.

The diskhd structure is used to manage a linked list of I/O requests.

struct diskhd {
    long     b_flags;         /* not used, needed for consistency*/
    struct   buf *b_forw,    *b_back;       /* queue of unit queues */
    struct   buf *av_forw,    *av_back;    /* queue of bufs for this unit */
    long     b_bcount;            /* active flag */
};

The diskhd data structure has two buf pointers that the driver can manipulate. The av_forw pointer points to the first active I/O request. The second pointer, av_back, points to the last active request on the list.

A pointer to this structure is passed as an argument to disksort(9F), along with a pointer to the current buf structure being processed. The disksort() routine sorts the buf requests to optimize disk seek. The routine then inserts the buf pointer into the diskhd list. The disksort() program uses the value that is in b_resid of the buf structure as a sort key. The driver is responsible for setting this value. Most Sun disk drivers use the cylinder group as the sort key. This approach optimizes the file system read-ahead accesses.

When data has been added to the diskhd list, the device needs to transfer the data. If the device is not busy processing a request, the xxstart() routine pulls the first buf structure off the diskhd list and starts a transfer.

If the device is busy, the driver should return from the xxstrategy() entry point. When the hardware is done with the data transfer, an interrupt is generated. The driver's interrupt routine is then called to service the device. After servicing the interrupt, the driver can then call the start() routine to process the next buf structure in the diskhd list.