hi, I'm studying xfs delayed logging codes, and currently have some questions about log reservation, can anyone help to answer below questions, thanks in advance! 1, what's the difference between xlog's l_reserve_head and l_write_head? Seems that l_reserve_head already can been used to do log space reservation, from codes, I really don't get when to use l_reserve_head or l_write_head, so what different cases are they used for? 2, what's the exact definition about permanent transaction reservation? In xfs_trans_resv_calc(), I see many kinds of transactions have XFS_TRANS_PERM_LOG_RES enabled, so non-permanent transaction does not need to do log reservation at the begin? 3, struct xfs_trans_res's tr_logcount(/* number of log operations per log ticket */) For exmaple, tr_write.tr_logcount is XFS_WRITE_LOG_COUNT_REFLINK(8), does that mean to complete a write operation, we'll need 8 log operations, such as file block mapping update intent will be counted one? 4, what's the exact definition about xfs rolling transactions? Is that because we use WAL log ing xfs, so for example, if we free a file extent, we at least create an extent free intent in one transaction, and when this transaction is committed to disk, then do the real xfs b+ tree modifications, finally once we completed the b+ tree modifications, we log an extent free done intent in one new transactino? so we'll need to reserve all the needed log space an the begin, and the whole process needs multiple transactions, so it's named rolling transactions? 5, finally are there any documents that describe the initial xfs log design before current delayed logging design? Documentation/filesystems/xfs-delayed-logging-design.rst is a very good document, but seems that it assumes that readers has been familiar with initial xfs log design. Regards, Xiaoguang Wang
