Weiping, > For virtio block device, actually there is no a hard limit for max > request size, and virtio_blk driver set -1 to > blk_queue_max_hw_sectors(q, -1U);. But it doesn't work, because there > is a default upper limitation BLK_DEF_MAX_SECTORS (1280 sectors).
That's intentional (although it's an ongoing debate what the actual value should be). > So this series want to add a new helper > blk_queue_max_hw_sectors_no_limit to set a proper max reqeust size. BLK_DEF_MAX_SECTORS is a kernel default empirically chosen to strike a decent balance between I/O latency and bandwidth. It sets an upper bound for filesystem requests only. Regardless of the capabilities of the block device driver and underlying hardware. You can override the limit on a per-device basis via max_sectors_kb in sysfs. People generally do it via a udev rule. -- Martin K. Petersen Oracle Linux Engineering
