On Tue 16-02-10 15:40:22, Kailas Joshi wrote: > On 15 February 2010 20:30, Jan Kara <jack@xxxxxxx> wrote: > > On Sat 13-02-10 14:13:17, Kailas Joshi wrote: > >> On 13 February 2010 01:37, <tytso@xxxxxxx> wrote: > >> > On Fri, Feb 12, 2010 at 08:52:15AM +0530, Kailas Joshi wrote: > >> >> 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. > >> Okay. So, basically process stalling is unavoidable as we cannot > >> modify a buffer data in past transaction after it has been modified in > >> current transaction. > >> Can we restrict the scope for this blocking? Blocking on > >> journal_start() will block all processes even though they are > >> operating on mutually exclusive sets of metadata buffers. Can we > >> restrict this blocking to allocation/deallocation paths by blocking in > >> get_write_access() on specific cases(some condition on buffer)? This > >> way, since all files will use commit-time allocation, very few(sync > >> and direct-io mode) file operations will be stalled. > > I doubt blocking at buffer-level would be enough. I think that the > > journalling layer just does not have enough information for such decisions. > > It could be feasible to block on per-inode basis but you'd still have to > > give a good thought to modification of filesystem global structures like > > bitmaps, superblock, or inode blocks. > Okay. So, blocking at buffer level will not be easy as global > structures shared among inodes will need modifications(for example, > changing access time for a file in inode block). Yes. > One last doubt, while looking at the code, I saw that journal_start() > always stalls all file operations while currently running transaction > is in LOCKED state. Only when the current transaction moves to FLUSH, > the new transaction is created and the stalled operations continue. Is > this interpretation correct? Yes, it is correct. > If yes, why this stalling does not have significant negative impact on > performance of file operations? Also, if it does not have, will > stalling for delayed block allocation really have such significant > negative impact? Actually, stalling on a transaction in LOCKED state does have a negative impact on the filesystem performance. But it's hard to avoid it. The transaction is in LOCKED state while we've decided it needs a commit but there are still tasks which have handle to it and are adding new metadata buffers to it. So this transaction is effectively still running and we cannot start a next transaction because then we'd have two running transactions and the journalling logic isn't able to handle that. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
