Hello Guys,
I found a interesting thing on GFS2 file system, After I did a direct IO write for a whole file, I still saw there were some page caches in this inode.
It looks this GFS2 behavior does not follow file system POSIX semantics, I just want to know this problem belongs to a know issue or we can fix it?
By the way, I did the same testing on EXT4 and OCFS2 file systems, the result looks OK.
I will paste my testing command lines and outputs as below,
For EXT4 file system,
tb-nd1:/mnt/ext4 # rm -rf f3
tb-nd1:/mnt/ext4 # dd if=/dev/urandom of=./f3 bs=1M count=4 oflag=direct
4+0 records in
4+0 records out
4194304 bytes (4.2 MB, 4.0 MiB) copied, 0.0393563 s, 107 MB/s
tb-nd1:/mnt/ext4 # vmtouch -v f3
f3
[ ] 0/1024
Files: 1
Directories: 0
Resident Pages: 0/1024 0/4M 0%
Elapsed: 0.000424 seconds
tb-nd1:/mnt/ext4 #
For OCFS2 file system,
tb-nd1:/mnt/ocfs2 # rm -rf f3
tb-nd1:/mnt/ocfs2 # dd if=/dev/urandom of=./f3 bs=1M count=4 oflag=direct
4+0 records in
4+0 records out
4194304 bytes (4.2 MB, 4.0 MiB) copied, 0.0592058 s, 70.8 MB/s
tb-nd1:/mnt/ocfs2 # vmtouch -v f3
f3
[ ] 0/1024
Files: 1
Directories: 0
Resident Pages: 0/1024 0/4M 0%
Elapsed: 0.000226 seconds
For GFS2 file system,
tb-nd1:/mnt/gfs2 # rm -rf f3
tb-nd1:/mnt/gfs2 # dd if=/dev/urandom of=./f3 bs=1M count=4 oflag=direct
4+0 records in
4+0 records out
4194304 bytes (4.2 MB, 4.0 MiB) copied, 0.0579509 s, 72.4 MB/s
tb-nd1:/mnt/gfs2 # vmtouch -v f3
f3
[ oo oOo ] 48/1024
Files: 1
Directories: 0
Resident Pages: 48/1024 192K/4M 4.69%
Elapsed: 0.000287 seconds
For vmtouch tool, you can download it's source code from https://github.com/hoytech/vmtouch
I also printk the inode's address_space after a full file direct-IO write in kernel space,
the nrpages value in the inode's address_space is always greater than zero.
Thanks
Gang
