On Tue, Apr 19, 2011 at 3:45 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > On Tue, Apr 19, 2011 at 02:53:20PM +0800, Yongqiang Yang wrote: >> On Tue, Apr 19, 2011 at 11:44 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: >> > On Tue, Apr 19, 2011 at 09:58:15AM +0800, Yongqiang Yang wrote: >> >> On Mon, Apr 18, 2011 at 10:45 AM, Andreas Dilger <adilger@xxxxxxxxx> wrote: >> >> > On 2011-04-17, at 6:40 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: >> >> > >> >> > On Sat, Apr 16, 2011 at 08:21:28AM -0400, Theodore Tso wrote: >> >> > >> >> > On Apr 16, 2011, at 1:11 AM, Andreas Dilger wrote: >> >> > >> >> > In that case, it means cp should just always use FIEMAP_FLAG_SYNC, which is >> >> > fine. >> >> > >> >> > Except that if someone is copying a large delay allocated file, it will >> >> > cause >> >> > >> >> > the file to immediately snapped to disk, which might not be the greatest >> >> > >> >> > thing in the world. >> >> > >> >> > Obvious workaround - if the initial fiemap call shows unwritten >> >> > extents, redo it with the sync flag set. Though that assumeÑ that >> >> > you can trust things like delalloc extents to only cover the range >> >> > that valid data exists in. Which, of course, you can't assume, >> >> > either. :/ >> >> > >> >> > Always passingÂFIEMAP_FLAG_SYNC is fine in this case. It should only do >> >> > anything if there is unwritten data, which is the only case we are concerned >> >> > with at this point. ÂIn any case, this is a simple solution for coreutils >> >> > until such a time that a more complex solution is added in the kernel (if >> >> > ever). >> >> > >> >> > Christoph is write, SEEK_HOLE and SEEK_DATA are >> >> > >> >> > a much better API for what cp woulld lke to do. ÂUnfortunately it hasn't >> >> > >> >> > been implemented yet in the VFS... >> >> > >> >> > Agreed, SEEK_HOLE/SEEK_DATA is the right way to solve this problem. >> >> > >> >> > I don't see how this will change the problem in any meaningful way. There >> >> > will still need to be code that is traversing the on-disk mapping, and also >> >> > keeping it coherent with unwritten data in the page cache. >> >> >> >> It seems that we are being messed up by page cache and disk. >> >> Unwritten flag returned from FIEMAP indicates blocks on disk are not >> >> written, but it does not say if there is data in page cache. ÂSo >> >> FIEMAP itself just tells user the map on disk. ÂHowever there is an >> >> exception for delayed allocation, ÂFIEMAP tells users the data is in >> >> page cache. >> > >> > No, FIEMAP does not tell the user there is data in the page cache. >> > It tells there user there is a delayed allocation extent. For XFS, a >> > delayed allocation extent can cover a range _greater_ than there is >> > data in the page cache - we do allocation allignment, speculative >> > allocation and other tricks to avoid fragmentation via >> > delayed allocation. When XFSs says there is a delalloc extent, it is >> > simply showing the in-memory representation of the extent. if you >> > want to know where the data in the page cache actually is, you need >> > to sync the file to disk to get those ranges converted to real >> > extents. This is how xfs_bmap has worked for 15 years.... >> > >> >> Maybe FIEMAP should return all known messages for unwritten extent, if >> >> unwritten data exists in page cache, FIEMAP should let users know that >> >> data is in page cache and space on disk has been preallocated, but >> >> data has not been flushed into disk. ÂActually, delayed allocation has >> >> done like this. Then user-space applications can determine how to do. >> >> Taking cp as an example, it will copy from page cache rather ignore >> >> it. >> > >> > Once again, FIEMAP is for showing the filesystem's current extent >> > state, not the page cache state. Ext4 may implement FIEMAP by doing >> > page cache walks, but that is a filesystem specific implementation >> > detail. >> > >> >> We need a definite definition for FIEMAP, in other words, it tells >> >> users map on disk or both disk and page cache. >> > >> > We already have a definition - and it has nothing to do with the >> > page cache state. >> > >> >> If the former one is taken, then FIEMAP should not consider >> >> delayed allocation. >> > >> > Not at all. the delayed allocation extent is a first class extent >> > type in XFS and it is reported directly from the extent list. Your >> > viewpoint is very ext4-specific and ignores the fact that other >> > filesystems were doing this sort of mapping long before even ext3 >> > (let alone ext4) was a glint in the designer's eye.... >> > >> >> otherwise, FIEMAP should return all known messages for unwritten case >> >> like delayed allocation. >> > >> > See my previous comments about extents being unwritten until data is >> > physically written to them. >> Understood, thank you for your explanation. >> >> Ok. ÂLet's look at it from a higher view. ÂWhat you described about >> extent state is specific to xfs. >> >> I think there are 2 ways to provide a definite definition for FIEMAP >> for all filesystems: >> >> 1. FIEMAP returns extent state on disk. >> 2. FIEMAP returns extent both in memory and on disk. > > You are *not listening*. There is no #2. FIEMAP returns the extent > state _on disk_ at the time of the call. If you want it to reflect > the in-memory state at the time of the call (for data or metadata), > you *must* use the the SYNC flag to convert that in-memory state to > on-disk state, which FIEMAP then reports just fine. Sorry for being dense. I think delayed extent is an exception. because it is not on the disk. Yongqiang. > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx > -- Best Wishes Yongqiang Yang -- 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
