chris.mason@oracle.com, Al Viro <viro@zeniv.linux.org.uk>, Ted Ts'o <tytso@mit.edu>

chris.mason@oracle.com, Al Viro <viro@zeniv.linux.org.uk>, Ted Ts'o <tytso@mit.edu>

[Jacek Luczak]

Hi All,

/*Sorry for sending incomplete email, hit wrong button :) */

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