Hi Neha On 5/13/13, neha naik <nehanaik27@gmail.com> wrote:
Hi, I was under the impression that when the bio comes to a block device driver its size is variable. But, after writing a block device driver i realized that such is not the case and it always is 4096.
Your Filesystem block size is usually page size so from file system point of view your bios would be that of block size that the filesystem understands. Try having a file system block size other than PAGE_SIZE (must be multiple of 512 and <=PAGE_SIZE) though.
Does this mean that by default if i don't use any merge function the bio will always have size 4096(meaning it will have only 1 page!!!!).
I guess it's not about that if you don't use a merge function. Bios would be given by file system to your driver depending on the block size of the file system. Now there would most probably be a mismatch of granularity of file system block size and your device's block size. If every thing is default and File System block size == PAGE_SIZE and by default the block size for your device is taken to be 512 (kernel always works in 512 bytes for block device). This means 4096/512 = 8 sectors per bio. Actually if you see this, --------> set_min_blocksize This would make it clear that the block size of the file system would be larger of the block size of block size of your device and the block size of your file system. I guess while formatting the mkfs would also try to match it like that or it may be fixed. You can change the block size of your device by doing blk_queue_logical_block_size before you make you add your disk. Again, if you do this your request handling function must be able to translate everything to/from 512 bytes since for kernel everything is in 512 size sectors. So you can check a) By changing the block size of file system first. b) By changing the block size of device. c) Do both By changing i mean don't use PAGE_SIZE for file system and don't use 512 bytes for device's block size.
Regards, Neha
---P.K.S -- Pranay Srivastava