Thank you very much for the reply. That clarifies a lot of things. I was trying a small test case that opens a file, writes a block of data, calls fsync and then closes the file. If I understand correctly, fsync would return only after all in-memory buffers have been committed to disk. I have added few print statements in the __extent_writepage function, and I notice that the function gets called a bit later after fsync returns. It seems that I am not guaranteed to see the data going to disk by the time fsync returns.
Am I doing something wrong, or am I looking at the wrong place for disk write? This happens both with tree logging enabled as well as with notreelog. Thanks On 29 September 2013 02:42, Josef Bacik <jba...@fusionio.com> wrote: > On Sun, Sep 29, 2013 at 01:35:15AM +0200, Aastha Mehta wrote: >> Hi, >> >> I have few questions regarding logging triggered by calling fsync in BTRFS: >> >> 1. If I understand correctly, fsync will call to log entire inode in >> the log tree. Does this mean that the data extents are also logged >> into the log tree? Are they copied into the log tree, or just >> referenced? Are they copied into the subvolume's extent tree again >> upon replay? >> > > The data extents are copied as well, as in the metadata that points to the > data, > not the actual data itself. For 3.1 it's all of the extents in the inode, in > 3.8 on it's only the extents that have changed this transaction. > >> 2. During replay, when the extents are added into the extent >> allocation tree, do they acquire the physical extent number during >> replay? Does they physical extent allocated to the data in the log >> tree differ from that in the subvolume? >> > > No the physical location was picked when we wrote the data out during fsync. > If > we crash and re-mount the replay will just insert the ref into the extent tree > for the disk offset as it replays the extents. > >> 3. I see there is a mount option of notreelog available. After >> disabling tree logging, does fsync still lead to flushing of buffers >> to the disk directly? >> > > notreelog just means that we write the data and wait on the ordered data > extents > and then commit the transaction. So you get the data for the inode you are > fsycning and all of the metadata for the entire file system that has changed > in > that transaction. > >> 4. Is it possible to selectively identify certain files in the log >> tree and flush them to disk directly, without waiting for the replay >> to do it? >> > > I don't understand this question, replay only happens on mount after a > crash/power loss, and everything is replayed that is in the log, there is no > way > to select which inode is replayed. Thanks, > > Josef -- Aastha Mehta MPI-SWS, Germany E-mail: aasth...@mpi-sws.org -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
