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During fiemap and lseek (hole and data seeking), there's no point in
iterating the inode's io tree to count delalloc bits if the inode's
delalloc bytes counter has a value of zero, as that counter is updated
whenever we set a range for delalloc or clear a range from delalloc.
So skip the counting and io tree iteration if the inode's delalloc bytes
counter has a value of zero. This helps save time when processing a file
range corresponding to a hole or prealloc (unwritten) extent.
This patch is part of a series comprised of the following patches:
btrfs: get the next extent map during fiemap/lseek more efficiently
btrfs: skip unnecessary extent map searches during fiemap and lseek
btrfs: skip unnecessary delalloc search during fiemap and lseek
The following test was performed on a release kernel (Debian's default
kernel config) before and after applying those 3 patches.
# Wrapper to call fiemap in extent count only mode.
# (struct fiemap::fm_extent_count set to 0)
$ cat fiemap.c
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <sys/ioctl.h>
#include <linux/fs.h>
#include <linux/fiemap.h>
int main(int argc, char **argv)
{
struct fiemap fiemap = { 0 };
int fd;
if (argc != 2) {
printf("usage: %s <path>\n", argv[0]);
return 1;
}
fd = open(argv[1], O_RDONLY);
if (fd < 0) {
fprintf(stderr, "error opening file: %s\n",
strerror(errno));
return 1;
}
/* fiemap.fm_extent_count set to 0, to count extents only. */
fiemap.fm_length = FIEMAP_MAX_OFFSET;
if (ioctl(fd, FS_IOC_FIEMAP, &fiemap) < 0) {
fprintf(stderr, "fiemap error: %s\n",
strerror(errno));
return 1;
}
close(fd);
printf("fm_mapped_extents = %d\n", fiemap.fm_mapped_extents);
return 0;
}
$ gcc -o fiemap fiemap.c
And the wrapper shell script that creates a file with many holes and runs
fiemap against it:
$ cat test.sh
#!/bin/bash
DEV=/dev/sdi
MNT=/mnt/sdi
mkfs.btrfs -f $DEV
mount $DEV $MNT
FILE_SIZE=$((1 * 1024 * 1024 * 1024))
echo -n > $MNT/foobar
for ((off = 0; off < $FILE_SIZE; off += 8192)); do
xfs_io -c "pwrite -S 0xab $off 4K" $MNT/foobar > /dev/null
done
# flush all delalloc
sync
start=$(date +%s%N)
./fiemap $MNT/foobar
end=$(date +%s%N)
dur=$(( (end - start) / 1000000 ))
echo "fiemap took $dur milliseconds"
umount $MNT
Result before applying patchset:
fm_mapped_extents = 131072
fiemap took 63 milliseconds
Result after applying patchset:
fm_mapped_extents = 131072
fiemap took 39 milliseconds (-38.1%)
Running the same test for a 512M file instead of a 1G file, gave the
following results.
Result before applying patchset:
fm_mapped_extents = 65536
fiemap took 29 milliseconds
Result after applying patchset:
fm_mapped_extents = 65536
fiemap took 20 milliseconds (-31.0%)
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
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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.