On Sat, Apr 16, 2011 at 02:05:51PM +0800, Yongqiang Yang wrote: > On Sat, Apr 16, 2011 at 8:50 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Thu, Apr 14, 2011 at 11:01:04PM -0600, Andreas Dilger wrote: > >> On 2011-04-14, at 6:09 PM, Dave Chinner <david@xxxxxxxxxxxxx> > >> wrote: > >> > No, this was explicitly laid out in the fiemap interface > >> > discussions - it's up to the applicaiton to decide if it needs > >> > to do a sync first. That's what the FIEMAP_FLAG_SYNC control > >> > flag is for. This forces the fiemap call to do a fsync _before_ > >> > getting the mapping. If you want to know the exact layout of the > >> > file is, then you must use this flag. > >> > > >> > Even so, it is recognised that this is racy - any use of the > >> > block map has a time-of-read-to-time-of-use race condition that > >> > means you have to _verify_ the copy after it completes. FYI, > >> > that's what xfs_fsr does when copying based on extent maps - if > >> > the inode has changed in _any way_ during the copy, it aborts > >> > the copy of that file. > >> > > >> > i.e. using fiemap for copying is at best a *hint* about the > >> > regions that need copying, and it is in no way a guarantee that > >> > you'll get all the information you need to make accurate copy > >> > even if you do use the synchronous variant. > >> > >> I would tend to agree with Pádraig. If there is data in the > >> mapping (regardless of whether it is on disk or not), the FIEMAP > >> should return this to the caller. The SYNC flag is only intended > >> to flush the data to disk for tools that are doing > >> direct-to-disk operations on the data. > > > > What you are suggesting is that FIEMAP needs to be page cache > > coherent, and that is far, far away from the intended use of the > > interface. Even consiering that you need to looking for active pages > > in the page cache when mapping extents say to me that you are > > doing something very wrong. > > > > Unwritten extents remain unwritten until the data is physically > > written to them. Therefore, to change their state, you need to sync > No, buffered writes change their state without sync. They shouldn't. > > the data covering the range. _Lying_ about whether an extent is in > > the unwritten state is a really bad precedence to set, especially as > > it is then guaranteed to change state when a crash occurs (Why did > > recovery zero out my file? FIEMAP said it contained data before my > > system crashed!). > > All filesystems have metadata in memory which is not flushed to > permanent storage. e.g. if a extent exists in memory, but itself and > corresponding data are not flushed to permanent storage. Sure, but in the case of unwritten extents, XFS does not change the metadata state in memory until *after the physical IO is completed*. I'm pretty sure that btrfs is the same. IOWs, despite the fact that a buffered write has occurred, no metadata has changed state in memory, and the extents are still unwritten in both memory and on disk.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- 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
