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. > 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. So you said above can only be achieved by sync before FIEMAP. Otherwise if a crash occurs, FIEMAP can not find data before system crashed. Without delayed allocation, there is no difference between preallocation case(fallocate) and normal cases. > Don't try to mangle the API semantics every time someone doesn't > understand how to use FIEMAP reliably. If you need the extent list > returned by FIEMAP to match what is in the page cache *regardless of > >> Otherwise the UNMAPPED flag is useless, since even with "check, >> copy, check" there is no guarantee that the inode is changed >> _during_ the copy operation. It could have been written into the >> cache _before_ the FIEMAP and remain unchanged and in your case >> there would be no way to know any data was ever written to the >> file without SYNC on ever single file before FIEMAP. > > I can't find any UNMAPPED flag in the FIEMAP interface, so I have no > idea what you are refering to here. > > 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 > -- Best Wishes Yongqiang Yang _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
