MisterX Tech

In my humble opinion when it comes to CLI tools `dd` ranks pretty high in my toolkit. Right there with `nc` and a few other ‘ole favorites.

Today I wanted to show a quick hack on how to determine information about a block device using dd. This goes along the same path as  previous post about using `dd` to get LVM configuration off devices.

Well enough talk lets get down to the CLI:

DD IS DESTRUCTIVE USE IT WITH CAUTION AND UNDERSTANDING OF WHAT YOU ARE RUNNING!…ok now that is out of the way…

# Thanks to file(1) magic(5)
# Example 1 - CentOS 6 LVM2
>dd if=/dev/sda3 of=/tmp/info bs=512 count=1000
1000+0 records in
1000+0 records out
512000 bytes (512 kB) copied, 0.0190352 s, 26.9 MB/s
You have new mail in /var/spool/mail/root
> file /tmp/info
/tmp/info: LVM2 (Linux Logical Volume Manager) , UUID: M7pUi7daFBsEQ95UNJUd5pN604qSa0Z


#Example 2 - CentOS 7 LVM2
> dd if=/dev/sda2 of=/tmp/info bs=4k count=100
100+0 records in
100+0 records out
409600 bytes (410 kB) copied, 0.00487443 s, 84.0 MB/s
> file /tmp/info
/tmp/info: LVM2 PV (Linux Logical Volume Manager), UUID: fYcfT0-3Oyk-J87h-A6SV-rYCc-q1of-zhx7Fd, size: 119508303872


#Example 3 - CentOS 7 XFS
> dd if=/dev/sda1 of=/tmp/info bs=4k count=100
100+0 records in
100+0 records out
409600 bytes (410 kB) copied, 0.00990939 s, 41.3 MB/s
> file /tmp/info
/tmp/info: SGI XFS filesystem data (blksz 4096, inosz 256, v2 dirs)


#Example 4 - CentOS 7 BTRFS
> dd if=/dev/vdb2 of=/tmp/info bs=4k count=100
100+0 records in
100+0 records out
409600 bytes (410 kB) copied, 0.00324626 s, 126 MB/s
> file /tmp/info
/tmp/info: BTRFS Filesystem (label "----_fs", sectorsize 4096, nodesize 16384, leafsize 16384)


#Example 5 - CentOS 6 - EXT4
> dd if=/dev/sda1 of=/tmp/info bs=4k count=100
100+0 records in
100+0 records out
409600 bytes (410 kB) copied, 0.0132642 s, 30.9 MB/s
> file /tmp/info
/tmp/info: Linux rev 1.0 ext4 filesystem data (needs journal recovery) (extents) (huge files)

I always forget to test this when I have more “esoteric” filesystems in my home lab. If anyone can and post a comment with the results from other filesystems’s or distros I would appreciate it!

This is more of a exercise than a real world use example. I have used something similar in a disater recovery situation so it does have some merit ..but good planning can prevent needing hacks like this.

DD IS DESTRUCTIVE USE IT WITH CAUTION AND UNDERSTANDING OF WHAT YOU ARE RUNNING!…ok now that is out of the way…

for i in $(pvdisplay | awk '/PV Name/ {print $3}'); do
dd if=$i of=/tmp/lvm_config_for_$(echo $i |sed 's/\//_/g') bs=$(stat -f -c %s $i) count=10;
done


This month we are checking out fallocate. This can replace dd as a superfast file creation tool. Here is a example:

fallocate -l 1G /tmp/test.2

For a quick benchmark (time output abbreviated for brevity):

/usr/bin/time -vvv fallocate -l 1G /tmp/test.2
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.00
Maximum resident set size (kbytes): 2192
Voluntary context switches: 1
Involuntary context switches: 1

/usr/bin/time -vvv dd if=/dev/zero of=/tmp/test.2 bs=1G count=1
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:26.15
Maximum resident set size (kbytes): 4197616
Voluntary context switches: 5023
Involuntary context switches: 864

(UPDATE)
I wanted to test fallocate on some non-standard filesystem like Lustre and finally got the chance and, like I had feared, it did not like it

fallocate -l 1G 1Gtest
fallocate: 1Gtest: fallocate failed: Operation not supported

but it did like the `dd if=/dev/zero of=/tmp/test.2 bs=1G count=0 seek=1` command given below in the comments section.

I have some ideas on why this does not work but I will save that for later….

INFO on Lustre:
$>lfs getstripe stripe8/
stripe8/
stripe_count: 8 stripe_size: 1048576 stripe_offset: -1

If anyone has a distributed filesystem like Gluster installed and can do some fallocate testing I would be interested in seeing if its broken there as well (Iwager it is). If not I might just get it going and play around with it.