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Subjectgetdents - ext4 vs btrfs performance
From
Hi All,

/*Sorry for sending incomplete email, hit wrong button :) I guess I
can't use Gmail */
Long story short: We've found that operations on a directory structure
holding many dirs takes ages on ext4.

The Question: Why there's that huge difference in ext4 and btrfs? See
below test results for real values.

Background: I had to backup a Jenkins directory holding workspace for
few projects which were co from svn (implies lot of extra .svn dirs).
The copy takes lot of time (at least more than I've expected) and
process was mostly in D (disk sleep). I've dig more and done some
extra test to see if this is not a regression on block/fs site. To
isolate the issue I've also performed same tests on btrfs.

Test environment configuration:
1) HW: HP ProLiant BL460 G6, 48 GB of memory, 2x 6 core Intel X5670 HT
enabled, Smart Array P410i, RAID 1 on top of 2x 10K RPM SAS HDDs.
2) Kernels: All tests were done on following kernels:
- 2.6.39.4-3 -- the build ID (3) is used here for internal tacking of
config changes mostly. In -3 we've introduced ,,fix readahead pipeline
break caused by block plug'' patch. Otherwise it's pure 2.6.39.4.
- 3.2.7 -- latest kernel at the time of testing (3.2.8 has been
release recently).
3) A subject of tests, directory holding:
- 54GB of data (measured on ext4)
- 1978149 files
- 844008 directories
4) Mount options:
- ext4 -- errors=remount-ro,noatime,
data=writeback
- btrfs -- noatime,nodatacow and for later investigation on
copression effect: noatime,nodatacow,compress=lzo

In all tests I've been measuring time of execution. Following tests
were performed:
- find . -type d
- find . -type f
- cp -a
- rm -rf

Ext4 results:
| Type | 2.6.39.4-3 | 3.2.7
| Dir cnt | 17m 40sec | 11m 20sec
| File cnt | 17m 36sec | 11m 22sec
| Copy | 1h 28m | 1h 27m
| Remove| 3m 43sec | 3m 38sec

Btrfs results (without lzo comression):
| Type | 2.6.39.4-3 | 3.2.7
| Dir cnt | 2m 22sec | 2m 21sec
| File cnt | 2m 26sec | 2m 23sec
| Copy | 36m 22sec | 39m 35sec
| Remove| 7m 51sec | 10m 43sec

From above one can see that copy takes close to 1h less on btrfs. I've
done strace counting times of calls, results are as follows (from
3.2.7):
1) Ext4 (only to elements):
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
57.01 13.257850 1 15082163 read
23.40 5.440353 3 1687702 getdents
6.15 1.430559 0 3672418 lstat
3.80 0.883767 0 13106961 write
2.32 0.539959 0 4794099 open
1.69 0.393589 0 843695 mkdir
1.28 0.296700 0 5637802 setxattr
0.80 0.186539 0 7325195 stat

2) Btrfs:
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
53.38 9.486210 1 15179751 read
11.38 2.021662 1 1688328 getdents
10.64 1.890234 0 4800317 open
6.83 1.213723 0 13201590 write
4.85 0.862731 0 5644314 setxattr
3.50 0.621194 1 844008 mkdir
2.75 0.489059 0 3675992 1 lstat
1.71 0.303544 0 5644314 llistxattr
1.50 0.265943 0 1978149 utimes
1.02 0.180585 0 5644314 844008 getxattr

On btrfs getdents takes much less time which prove the bottleneck in
copy time on ext4 is this syscall. In 2.6.39.4 it shows even less time
for getdents:
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
50.77 10.978816 1 15033132 read
14.46 3.125996 1 4733589 open
7.15 1.546311 0 5566988 setxattr
5.89 1.273845 0 3626505 lstat
5.81 1.255858 1 1667050 getdents
5.66 1.224403 0 13083022 write
3.40 0.735114 1 833371 mkdir
1.96 0.424881 0 5566988 llistxattr


Why so huge difference in the getdents timings?

-Jacek


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