Limiting the incoming Block I/O requests to a device driver/md device

When implementing a device driver or a MD device which can receive Block I/O (struct bio in the kernel), you can receive BIO of nearly any size, with any number of segments (segments are discontinued parts of a common buffer, defined in a bio request). You may want to limit :

– The number of segments you can receive with
[code lang=”c”]blk_queue_max_segments(queue, X);[/code] Where X is the number of segments per struct bio

– The maximal size of the request :
[code lang=”c”]blk_queue_max_hw_sectors(queue, Y);[/code] Where Y is the maximal size in sectors

For a md device, the queue can be recovered with mddev->queue

The combination of the two allows to limit ensure that all bio request have always maximum X segments for a maximal size of Y sectors.

It is used in raid0 with Y=mddev->chunk_sectors to ensure that no request is bigger than one chunk, so any request cross at most one chunk boundary. And with X=1, it allows to use the bio_split function to split a request which would span on the two sides of a chunk boundary.