Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > > With ->fiemap() you can at least make the distinction between a non > > > existing and an UNWRITTEN extent. > > > > I can't use that for XFS, Ext4 or btrfs, I suspect. Christoph and Dave's > > assertion is that the cache can't rely on the backing filesystem's metadata > > because these can arbitrarily insert or remove blocks of zeros to bridge or > > split extents. > > Well, that's not the big problem. The issue that makes FIEMAP > unusable for determining if there is user data present in a file is > that on-disk extent maps aren't exactly coherent with in-memory user > data state. > > That is, we can have a hole on disk with delalloc user data in > memory. There's user data in the file, just not on disk. Same goes > for unwritten extents - there can be dirty data in memory over an > unwritten extent, and it won't get converted to written until the > data is written back and the filesystem runs a conversion > transaction. > > So, yeah, if you use FIEMAP to determine where data lies in a file > that is being actively modified, you're going get corrupt data > sooner rather than later. SEEK_HOLE/DATA are coherent with in > memory user data, so don't have this problem. I thought you and/or Christoph said it *was* a problem to use the backing filesystem's metadata to track presence of data in the cache because the filesystem (or its tools) can arbitrarily insert blocks of zeros to bridge/break up extents. If that is the case, then that is a big problem, and SEEK_HOLE/DATA won't suffice. If it's not a problem - maybe if I can set a mark on a file to tell the filesystem and tools not to do that - then that would obviate the need for me to store my own maps. David
