The Red Hat Enterprise Linux 4 Update 6 kernel exhibits a large drop in sequential write performance primarily to a block device

Issue

• This is a performance regression from RHEL4.5 in single stream sequential write workloads when using the default (CFQ) I/O scheduler.
• The problem is restricted to performing I/O to a block device or an ext2 filesystem.
• The problem is not present when using the ext3 file system.

For block device

Warning: this will overwrite any existing data on the block device:

RHEL 4.5

  # time dd if=/dev/zero of=/dev/sdb1 bs=1M count=3000
  3000+0 records in
  3000+0 records out

  real    0m36.969s
  user    0m0.013s
  sys     0m13.224s

RHEL 4.6

  # time dd if=/dev/zero of=/dev/sdb1 bs=1M count=3000
  3000+0 records in
  3000+0 records out

  real    1m6.073s
  user    0m0.014s
  sys     0m8.659s

For an ext2 filesystem

Warning: this will overwrite the file "testfile" in the working  directory, if it exists:

RHEL 4.5

  # time dd if=/dev/zero of=testfile bs=1M count=3000
  3000+0 records in
  3000+0 records out

  real    0m39.362s
  user    0m0.017s
  sys     0m9.689s

RHEL4.6

  # time dd if=/dev/zero of=testfile bs=1M count=3000
  3000+0 records in
  3000+0 records out

  real    0m49.116s
  user    0m0.011s
  sys     0m9.231s