[ale] Understanding dd and/or /dev/zero

[Greg Freemyer]

I've noticed that I get a log of disk read activitity on /dev/hdc when I do 

    dd if=/dev/zero of=/dev/hdc

I'm using "iostat -d 5" to see the read activity, and it is really
slowing down the above.

I have no idea why there is disk read activity, and the machine is
doing nothing else.  Also if I kill the above dd, the read activity
stops.

Trying to understand it, I did an "strace dd  if=/dev/zero
of=/dev/hdc" and the initial part of the output is below.

I don't see anything overly strange, and once the repeating read/write
sequence begins it is always read /dev/zero, write /dev/hdc just as
one would expect.

Does anyone know what is causing the disk read activity?

==========>  strace of dd

execve("/bin/dd", ["dd", "if=/dev/zero", "of=/dev/hdc"], [/* 51 vars */]) = 0
uname({sys="Linux", node="Forensic-2", ...}) = 0
brk(0)                                  = 0x8050000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=76361, ...}) = 0
old_mmap(NULL, 76361, PROT_READ, MAP_PRIVATE, 3, 0) = 0x40018000
close(3)                                = 0
open("/lib/tls/libc.so.6", O_RDONLY)    = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\0L\1\000"...,
512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1359489, ...}) = 0
old_mmap(NULL, 1137708, PROT_READ|PROT_EXEC,
MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x4002b000
madvise(0x4002b000, 1137708, MADV_SEQUENTIAL|0x1) = 0
mprotect(0x4013a000, 27692, PROT_NONE)  = 0
old_mmap(0x4013b000, 16384, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x10f000) = 0x4013b000
old_mmap(0x4013f000, 7212, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x4013f000
close(3)                                = 0
old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
-1, 0) = 0x40141000
mprotect(0x4013b000, 4096, PROT_READ)   = 0
set_thread_area({entry_number:-1 -> 6, base_addr:0x40141460,
limit:1048575, seg_32bit:1, contents:0, read_exec_only:0,
limit_in_pages:1, seg_not_present:0, useable:1}) = 0
munmap(0x40018000, 76361)               = 0
brk(0)                                  = 0x8050000
brk(0x8071000)                          = 0x8071000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_LARGEFILE) = -1
ENOENT (No such file or directory)
open("/usr/share/locale/locale.alias", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=2528, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x40018000
read(3, "# Locale name alias data base.\n#"..., 4096) = 2528
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x40018000, 4096)                = 0
open("/usr/lib/locale/en_US.UTF-8/LC_CTYPE", O_RDONLY) = -1 ENOENT (No
such file or directory)
open("/usr/lib/locale/en_US.utf8/LC_CTYPE", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=208464, ...}) = 0
mmap2(NULL, 208464, PROT_READ, MAP_PRIVATE, 3, 0) = 0x40142000
close(3)                                = 0
open("/usr/lib/gconv/gconv-modules.cache", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=21544, ...}) = 0
mmap2(NULL, 21544, PROT_READ, MAP_SHARED, 3, 0) = 0x40175000
close(3)                                = 0
close(0)                                = 0
open("/dev/zero", O_RDONLY|O_LARGEFILE) = 0
close(1)                                = 0
open("/dev/hdc", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 1
rt_sigaction(SIGINT, NULL, {SIG_DFL}, 8) = 0
rt_sigaction(SIGINT, {0x80499f0, [], 0}, NULL, 8) = 0
rt_sigaction(SIGQUIT, NULL, {SIG_DFL}, 8) = 0
rt_sigaction(SIGQUIT, {0x80499f0, [], 0}, NULL, 8) = 0
rt_sigaction(SIGPIPE, NULL, {SIG_DFL}, 8) = 0
rt_sigaction(SIGPIPE, {0x80499f0, [], 0}, NULL, 8) = 0
rt_sigaction(SIGUSR1, NULL, {SIG_DFL}, 8) = 0
rt_sigaction(SIGUSR1, {0x80494d0, [], 0}, NULL, 8) = 0
read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 512) = 512
write(1, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 512) = 512
read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 512) = 512
write(1, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 512) = 512
read(0, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 512) = 512
write(1, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 512) = 512

===== end of strace

Thanks
Greg
-- 
Greg Freemyer