On Fri, Feb 12, 2010 at 08:52:15AM +0530, Kailas Joshi wrote: > Won't this get fixed by performing early reservations as mentioned in > my scheme? We are reserving required credits in the path of write > system call and these will be kept reserved until transaction commit. > So, the journal space for allocation at commit will be guaranteed. Yes, if you account for these separately. One challenge is over-estimating the needed credits will be tricky. If we go down this path, be sure that the bonnie style write(fd, &ch, 1) in a tight loop doesn't end up reserving a separate set of credits for each write system call to the same block. (It can be done; if the DA block is already instantiated, you can assume that credits have already been reserved.) > Sorry, I didn't understand why processes need to be suspended. > In my scheme, I am issuing magic handle only after locking the current > transaction. AFAIK after the transaction is locked, it can receive the > block journaling requests for already created handles(in our case, for > already reserved journal space), and the new concurrent requests for > journal_start() will go to the new current transaction. Since, the > credits for locked transaction are fixed (by means of early > reservations) we can know whether journal has enough space for the new > journal_start(). So, as long as journal has enough space available, > new processes need now be stalled. But while you are modifying blocks that need to go into the journal via the locked (old) transaction, it's not safe to start a new transaction and start issuing handles against the new transaction. Just to give one example, suppose we need to update the extent allocation tree for an inode in the locked/committing transaction as the delayed allocation blocks are being resolved --- and in another process, that inode is getting truncated or unlinked, which also needs to modify the extent allocation tree? Hilarty ensues, unless you use a block all attempts to create a new handle (practically speaking, by blocking all attempts to start a new transaction), until this new delayed allocation resolution phase which you have proposed is complete. - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
