On Mar 07, 2008 17:01 +0530, Aneesh Kumar K.V wrote: > On Fri, Mar 07, 2008 at 03:17:33AM -0800, Mingming Cao wrote: > > How about we start a journal with estimated worse case transaction > > credits and then take the i_data_sem down? So that we could ensure that > > whenever the i_data_sem is hold, the i_data is protected. That is what > > currently DIO does, I think. It would be nice to avoid introducing > > another semaphore to protect i_data for migration if we could. > > Estimating transaction for a single page directIO write may be easy. But > in case of migrate it involves new blocks allocated to carry the extents > and also we free the indirect blocks of ext3 and that would involve > update of bitmap from different groups. I am not sure we will be able to > come up with a value. But if yes and if we can get that many credits > from journal i agree that would be better than introducing a new > semaphore. Agreed - and if we have a generic routine to calculate the journal credits needed for a full-file (or better a range) indirect block operation (including bitmaps, group descriptors, and [dt]indirect blocks). I don't think there would be a serious failure case if it wasn't possible to convert a block-mapped file to extent-mapped while it was mmapped. At worst the administrator would need to do that some time later, or after a system reboot, so long as the conversion actually failed if the file had any mmaps. If this same requirement is introduced when we get defrag for ext4 (because the block mapping is changing on the file) then we may have to reconsider the benefits of the more complex code. Note we can also use the "journal credits needed" for fixing truncate in a similar manner to do it all in a single transaction to avoid zeroing all of the indirect blocks. All that would be needed for trunate is to call the above function, update the on-disk i_size, possibly zero out the partially-truncated block, and update the group descriptors and bitmaps. That would also allow "undelete" to work on ext3 again because the inode i_blocks and indirect blocks wouldn't be zeroed out anymore, like it was in ext2. Cheers, Andreas -- Andreas Dilger Sr. Staff Engineer, Lustre Group Sun Microsystems of Canada, Inc. -- 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
