btrfs: 21 minutes to read 1.2M file directory

[Andy Isaacson <adi@hexapodia.org>]

I have a directory with 1.2M files in it, which makes readdir very slow
on btrfs with cold caches (although it's reasonably fast with hot caches
as in the first example below):

% time find /btr/foo > /btr/foo.list
find /btr/foo > /btr/foo.list  4.10s user 7.97s system 36% cpu 33.275 total
% head /btr/foo.list
/btr/foo
/btr/foo/1281373625.777.fg.jpg
/btr/foo/1281373625.777.bg.jpg
/btr/foo/1281373625.948.fg.jpg
/btr/foo/1281373625.948.bg.jpg
/btr/foo/1281373626.096.fg.jpg
/btr/foo/1281373626.096.bg.jpg
/btr/foo/1281373626.218.fg.jpg
/btr/foo/1281373626.218.bg.jpg
/btr/foo/1281373626.350.fg.jpg
% wc !$
wc /btr/foo.list
 12166666  12166666 401499940 /btr/foo.list
% wc -l /btr/foo.list
12166666 /btr/foo.list
% sudo sysctl -w vm.drop_caches=3 vm.drop_caches=0
vm.drop_caches = 3
vm.drop_caches = 0
% time find /btr/foo > /btr/foo.list.2
find /btr/foo > /btr/foo.list.2  5.62s user 24.54s system 2% cpu 21:40.90 total
% uname -a
Linux pyron 2.6.36-rc7-00149-g29979aa #71 SMP Wed Oct 13 09:42:57 PDT 2010 x86_64 GNU/Linux

Interestingly, while readdir is busy I'm only seeing IO on sdb even
though the btrfs is on 3 targets:

Label: btr  uuid: 1271de53-b3d2-4d68-9d48-b19487e1c982
        Total devices 3 FS bytes used 555.13GB
        devid    1 size 18.65GB used 18.64GB path /dev/sda2
        devid    3 size 512.00GB used 44.13GB path /dev/sdc1
        devid    2 size 512.00GB used 511.76GB path /dev/sdb1

"iostat -k 1 | grep sdb" tells me:

Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn

sdb             173.00       692.00         0.00        692          0
sdb             185.00       740.00         0.00        740          0
sdb             198.00       792.00         0.00        792          0
sdb             177.00       712.00         0.00        712          0

I updated to a recent git and it's still slow (my test hasn't completed
yet 19 minutes in):

Linux pyron 2.6.37-rc6-11882-g55ec86f #72 SMP Mon Dec 20 13:34:38 PST 2010 x86_64 GNU/Linux

The devices are:

[    1.834527] ata1.00: ATA-7: INTEL SSDSA2M040G2GC, 2CV102HD, max UDMA/133
[    1.834816] ata1.00: 78165360 sectors, multi 1: LBA48 NCQ (depth 31/32)
[    1.835369] ata1.00: configured for UDMA/133
[    1.835776] scsi 0:0:0:0: Direct-Access     ATA      INTEL SSDSA2M040 2CV1 PQ: 0 ANSI: 5
...
[    2.904919] ata3.00: ATA-8: ST31500341AS, CC1H, max UDMA/133
[    2.905206] ata3.00: 2930277168 sectors, multi 0: LBA48 NCQ (depth 31/32)
[    2.947393] ata3.00: configured for UDMA/133
[    2.947850] scsi 2:0:0:0: Direct-Access     ATA      ST31500341AS     CC1H PQ: 0 ANSI: 5
...
[    3.989664] ata5: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[    4.018524] ata5.00: ATA-8: ST31500341AS, CC1H, max UDMA/133
[    4.018811] ata5.00: 2930277168 sectors, multi 0: LBA48 NCQ (depth 31/32)
[    4.060838] ata5.00: configured for UDMA/133
[    4.061205] scsi 4:0:0:0: Direct-Access     ATA      ST31500341AS     CC1H PQ: 0 ANSI: 5

The host is a "Intel(R) Core(TM) i7 CPU 930 @2.80GHz" with 12GB RAM.

Thanks,
-andy