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During fiemap, when checking if a data extent is shared we are doing the backref walking even if we already know the leaf is shared, which is a waste of time since if the leaf shared then the data extent is also shared. So skip the backref walking when we know we are in a shared leaf. The following test was measures the gains for a case where all leaves are shared due to a snapshot: $ cat test.sh #!/bin/bash DEV=/dev/sdj MNT=/mnt/sdj umount $DEV &> /dev/null mkfs.btrfs -f $DEV # Use compression to quickly create files with a lot of extents # (each with a size of 128K). mount -o compress=lzo $DEV $MNT # 40G gives 327680 extents, each with a size of 128K. xfs_io -f -c "pwrite -S 0xab -b 1M 0 40G" $MNT/foobar # Add some more files to increase the size of the fs and extent # trees (in the real world there's a lot of files and extents # from other files). xfs_io -f -c "pwrite -S 0xcd -b 1M 0 20G" $MNT/file1 xfs_io -f -c "pwrite -S 0xef -b 1M 0 20G" $MNT/file2 xfs_io -f -c "pwrite -S 0x73 -b 1M 0 20G" $MNT/file3 # Create a snapshot so all the extents become indirectly shared # through subtrees, with a generation less than or equals to the # generation used to create the snapshot. btrfs subvolume snapshot -r $MNT $MNT/snap1 # Unmount and mount again to clear cached metadata. umount $MNT mount -o compress=lzo $DEV $MNT start=$(date +%s%N) # The filefrag tool uses the fiemap ioctl. filefrag $MNT/foobar end=$(date +%s%N) dur=$(( (end - start) / 1000000 )) echo "fiemap took $dur milliseconds (metadata not cached)" echo start=$(date +%s%N) filefrag $MNT/foobar end=$(date +%s%N) dur=$(( (end - start) / 1000000 )) echo "fiemap took $dur milliseconds (metadata cached)" umount $MNT The results were the following on a non-debug kernel (Debian's default kernel config). Before this patch: (...) /mnt/sdi/foobar: 327680 extents found fiemap took 1821 milliseconds (metadata not cached) /mnt/sdi/foobar: 327680 extents found fiemap took 399 milliseconds (metadata cached) After this patch: (...) /mnt/sdi/foobar: 327680 extents found fiemap took 591 milliseconds (metadata not cached) /mnt/sdi/foobar: 327680 extents found fiemap took 123 milliseconds (metadata cached) That's a speedup of 3.1x and 3.2x. Signed-off-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.