On Thu, Dec 29, 2011 at 07:42:31PM +0530, Vijay Chauhan wrote: > Hi list, > > Can anyone please provide me links about the basic explanations of XFS > basic blocks and filesystem logical block mapping and conversion > routines? > > I tried to understand from code but its not clear to me: > #define XFS_FSB_TO_BB(mp,fsbno) ((fsbno) << (mp)->m_blkbb_log) Convert FileSystem Blocks to Basic Blocks FSBs are defined at mkfs time, BBs are always 512 bytes. > #define XFS_BB_TO_FSB(mp,bb) \ > (((bb) + (XFS_FSB_TO_BB(mp,1) - 1)) >> (mp)->m_blkbb_log) Convert BB to FSB, rounding up to the next FSB > #define XFS_BB_TO_FSBT(mp,bb) ((bb) >> (mp)->m_blkbb_log) Convert BB to FSB, rounding down to the FSB containing the BB > #define XFS_BB_FSB_OFFSET(mp,bb) ((bb) & ((mp)->m_bsize - 1)) Offset of the give BB within a FSB. e.g. if FSB = 4k = 8BB, then "bb = 5" would return 5, "bb = 63" would return 7... > lets consider the last one: > #define XFS_BB_FSB_OFFSET(mp,bb) ((bb) & ((mp)->m_bsize - 1)) > > if basic block (512 byte size) number is 7 and m_bsize is 12 m_bsize is the FSB in BB, which will always be a power of 2. > (considering FS block size 4096), then this will return 3 [e.g. ( 7 & in that case, m_bsize = 8. > 11) ]. then what does 3 means here? what offset value it is denoting? It's the offset of the BB within the first partial FSB in the range given. This was once used for sub-block zeroing needed by direct IO, but is now stale code as the generic DIO layer does this zeroing. Care to submit a patch to remove that macro? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
