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Re: [dm-devel] [PATCH] fix default IO restrictions



Christophe Saout wrote:
Am Mo, den 26.01.2004 schrieb Mike Christie um 22:22:


I don't think that will happen. The values are already the allowed
maximum values. Or am I wrong?

I don't think they are the max values. SCSI will override them depending on the HBA settings.


Hmm. Then I think there is something wrong anyway. Why are the max
constants not maximum values...


I think...


Why does blkdev.h know the max values? You are saying that they reflect a limitation of the kernel?

The queue's max_sectors, max_[phys/hw]_segments, max_segments_size etc are device driver/HW limitations. blk_init_queue does not know an optimal value for every device, but it might be able to set a safe default value. Maybe a better name for MAX_SECTORS is DEFAULT_MAX_SECTORS.

BIO_MAX_[SECTORS/SIZE/PAGES] is what you called a "system-wide limit". A bio can only be so large (the bvec_pool is hardcoded). However when merging the bio, q->max_sectors is one of the indicators that tell the queue when a request is too large for the device.



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