On Thu, Jul 13, 2017 at 04:16:44PM +0200, Aleksandar Čekrlić wrote: > 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. > Btrfs does support O_DIRECT, it is using 4K as its direct IO alignment (at least on x86_64), so the above dd may just use 512 and btrfs dio read falls back to buffered read then. thank, -liubo > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
