Hi all,
I am currently working on a file system related code where user can
opt to use direct I/O for file reads / writes. Code seems to be
working and operational, and on majority of the file systems I/O is
not using the page cache, but on BTRFS even though file is opened with
O_DIRECT file is cached, and subsequent reads are quite faster,
indicating that file read is not actually read from disk. I have
checked both with a separate binary (basically just open() with
O_DIRECT and read()) and with dd, and I get the same results.
dd reading a 200 MB file:
=================================
[root@lepton btrfs_mail]# linux-fincore /btrfs/smallfiles/dfile
filename
size total_pages min_cached page
cached_pages cached_size cached_perc
--------
---- ----------- ---------------
------------ ----------- -----------
/btrfs/smallfiles/dfile
209,715,200 51,200 -1
0 0 0.00
---
total cached size: 0
[root@lepton btrfs_mail]# dd if=/btrfs/smallfiles/dfile iflag=direct
of=/dev/null
409600+0 records in
409600+0 records out
209715200 bytes (210 MB) copied, 2.32787 s, 90.1 MB/s
[root@lepton btrfs_mail]# linux-fincore /btrfs/smallfiles/dfile
filename
size total_pages min_cached page
cached_pages cached_size cached_perc
--------
---- ----------- ---------------
------------ ----------- -----------
/btrfs/smallfiles/dfile
209,715,200 51,200 0
51,200 209,715,200 100.00
---
total cached size: 209,715,200
[root@lepton btrfs_mail]# dd if=/btrfs/smallfiles/dfile iflag=direct
of=/dev/null
409600+0 records in
409600+0 records out
209715200 bytes (210 MB) copied, 0.460326 s, 456 MB/s
=================================
I know in case the file system does not support O_DIRECT it will
ignore it, but I thought BTRFS does support it, so I'm kinda confused
by the behaviour.
System information (dmesg.log is attached) :
=================================
uname -a:
Linux lepton 2.6.32-642.el6.x86_64 #1 SMP Wed Apr 13 00:51:26 EDT 2016
x86_64 x86_64 x86_64 GNU/Linux
=================================
btrfs --version
Btrfs Btrfs v0.20-rc1
=================================
btrfs fi show
failed to stat /dev/VxDMP2
failed to stat /dev/VxDMP2p1
failed to stat /dev/VxDMP2p2
failed to stat /dev/VxDMP3
failed to stat /dev/VxDMP3p3
failed to stat /dev/VxDMP3p8
failed to stat /dev/VxDMP4
failed to stat /dev/VxDMP4p1
failed to stat /dev/VxDMP4p2
failed to stat /dev/VxDMP4p3
failed to stat /dev/VxDMP5
failed to stat /dev/VxDMP5p1
failed to stat /dev/VxDMP5p2
failed to stat /dev/VxDMP5p3
failed to stat /dev/VxVM21000
Label: none uuid: 903217cd-80e3-497e-8724-6e697d6c5d3e
Total devices 1 FS bytes used 620.84MB
devid 1 size 50.00GB used 6.04GB path /dev/cciss/c0d1p1
Btrfs Btrfs v0.20-rc1
=================================
btrfs fi df /btrfs
Data: total=2.01GB, used=619.95MB
System, DUP: total=8.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=2.00GB, used=900.00KB
Metadata: total=8.00MB, used=0.00
=================================
Thanks and best regards,
Aleksandar
Attachment:
dmesg.log
Description: Binary data
