Posts Tagged ‘ssh ftp’

How to Copy large data directories between 2 Linux / Unix servers without direct ssh / ftp access between server1 and server2 other by using SSH, TAR and Unix pipes

Monday, April 27th, 2015

Reading Time: 3minutes

how-to-copy-large-data-directories-between-2-linux-unix-servers-without-direct-ssh-ftp-access-btween-each-other

In a Web application data migration project, I've come across a situation where I have to copy / transfer 500 Gigabytes of data from Linux server 1 (host A) to Linux server 2 (host B). However the two machines doesn't have direct access to each other (via port 22) for security reasons and hence I cannot use sshfs to mount remotely host dir via ssh and copy files like local ones.

As this is a data migration project its however necessery to migrate the data finding a way … Normal way companies do it is to copy the data to External Hard disk storage and send it via some Country Post services or some employee being send in Data center to attach the SAN to new server where data is being migrated However in my case this was not possible so I had to do it different.

I have access to both servers as they're situated in the same Corporate DMZ network and I can thus access both UNIX machines via SSH.

Thanksfully there is a small SSH protocol + TAR archiver and default UNIX pipe's capabilities hack that makes possible to transfer easy multiple (large) files and directories. The only requirement to use this nice trick is to have SSH client installed on the middle host from which you can access via SSH protocol Server1 (from where data is migrated) and Server2 (where data will be migrated).

If the hopping / jump server from which you're allowed to have access to Linux  servers Server1 and Server2 is not Linux and you're missing the SSH client and don't have access on Win host to install anything on it just use portable mobaxterm (as it have Cygwin SSH client embedded )

Here is how:
 

jump-host:~$ ssh server1 "tar czf – /somedir/" | pv | ssh server2 "cd /somedir/; tar xf


As you can see from above command line example an SSH is made to server1  a tar is used to archive the directory / directories containing my hundred of gigabytes and then this is passed to another opened ssh session to server 2  via UNIX Pipe mechanism and then TAR archiver is used second time to unarchive previously passed archived content. pv command which is in the middle is not obligitory though it is a nice way to monitor status about data transfer like below:
 

500GB 0:00:01 [10,5MB/s] [===================================================>] 27%


P.S. If you don't have PV installed install it either with apt-get on Debian:

 

debian:~# apt-get install –yes pv

 

Or on CentOS / Fedora / RHEL etc.

 

[root@centos ~]# yum -y install pv

 

Below is a small chunk of PV manual to give you better idea of what it does:

NAME
       pv – monitor the progress of data through a pipe

SYNOPSIS
       pv [OPTION] [FILE]…
       pv [-h|-V]

DESCRIPTION
       pv  allows  a  user to see the progress of data through a pipeline, by giving information such as time elapsed, percentage
       completed (with progress bar), current throughput rate, total data transferred, and ETA.

       To use it, insert it in a pipeline between two processes, with the appropriate options.  Its standard input will be passed
       through to its standard output and progress will be shown on standard error.

       pv  will  copy  each  supplied FILE in turn to standard output (- means standard input), or if no FILEs are specified just
       standard input is copied. This is the same behaviour as cat(1).

       A simple example to watch how quickly a file is transferred using nc(1):

              pv file | nc -w 1 somewhere.com 3000

       A similar example, transferring a file from another process and passing the expected size to pv:

              cat file | pv -s 12345 | nc -w 1 somewhere.com 3000


Note that with too big file transfers using PV will delay data transfer because everything will have to pass through another 2 pipes, however for file transfers up to few gigabytes its really nice to include it.

If you only need to transfer huge .tar.gz archive and you don't bother about traffic security (i.e. don't care whether transferred traffic is going through encrypted SSH tunnel and don't want to put an overhead to both systems for encrypting the data and you have some unfiltered ports between host 1 and host 2 you can run netcat on host 2 to listen for connections and forward .tar.gz content via netcat's port like so:
 

linux2:~$ nc -l -p 12345 > /path/destinationfile
linux2:~$ cat /path/sourcfile | nc desti.nation.ip.address 12345


Another way to transfer large data without having connection with server1 and server2 but having connection to a third host PC is to use rsync and good old SSH Tunneling, like so:
 

jump-host:~$ ssh -R 2200:Linux-server1:22 root@Linux-server2 "rsync -e 'ssh -p 2200' –stats –progress -vaz /directory/to/copy root@localhost:/copy/destination/dir"

Auto restart Apache on High server load (bash shell script) – Fixing Apache server temporal overload issues

Saturday, March 24th, 2012

Reading Time: 4minutes

auto-restart-apache-on-high-load-bash-shell-script-fixing-apache-temporal-overload-issues

I've written a tiny script to check and restart, Apache if the server encounters, extremely high load avarage like for instance more than (>25). Below is an example of a server reaching a very high load avarage:;

server~:# uptime
13:46:59 up 2 days, 18:54, 1 user, load average: 58.09, 59.08, 60.05
load average: 0.09, 0.08, 0.08

Sometimes high load avarage is not a problem, as the server might have a very powerful hardware. A high load numbers is not always an indicator for a serious problems. Some 16 CPU dual core (2.18 Ghz) machine with 16GB of ram could probably work normally with a high load avarage like in the example. Anyhow as most servers are not so powerful having such a high load avarage, makes the machine hardly do its job routine.

In my specific, case one of our Debian Linux servers is periodically reaching to a very high load level numbers. When this happens the Apache webserver is often incapable to serve its incoming requests and starts lagging for clients. The only work-around is to stop the Apache server for a couple of seconds (10 or 20 seconds) and then start it again once the load avarage has dropped to less than "3".

If this temporary fix is not applied on time, the server load gets increased exponentially until all the server services (ssh, ftp … whatever) stop responding normally to requests and the server completely hangs …

Often this server overloads, are occuring at night time so I'm not logged in on the server and one such unexpected overload makes the server unreachable for hours.
To get around the sudden high periodic load avarage server increase, I've written a tiny bash script to monitor, the server load avarage and initiate an Apache server stop and start with a few seconds delay in between.

#!/bin/sh
# script to check server for extremely high load and restart Apache if the condition is matched
check=`cat /proc/loadavg | sed 's/\./ /' | awk '{print $1}'`
# define max load avarage when script is triggered
max_load='25'
# log file
high_load_log='/var/log/apache_high_load_restart.log';
# location of inidex.php to overwrite with temporary message
index_php_loc='/home/site/www/index.php';
# location to Apache init script
apache_init='/etc/init.d/apache2';
#
site_maintenance_msg="Site Maintenance in progress - We will be back online in a minute";
if [ $check -gt "$max_load" ]; then>
#25 is load average on 5 minutes
cp -rpf $index_php_loc $index_php_loc.bak_ap
echo "$site_maintenance_msg" > $index_php_loc
sleep 15;
if [ $check -gt "$max_load" ]; then
$apache_init stop
sleep 5;
$apache_init restart
echo "$(date) : Apache Restart due to excessive load | $check |" >> $high_load_log;
cp -rpf $index_php_loc.bak_ap $index_php_loc
fi
fi

The idea of the script is partially based on a forum thread – Auto Restart Apache on High Loadhttp://www.webhostingtalk.com/showthread.php?t=971304Here is a link to my restart_apache_on_high_load.sh script

The script is written in a way that it makes two "if" condition check ups, to assure 100% there is a constant high load avarage and not just a temporal 5 seconds load avarage jump. Once the first if is matched, the script first tries to reduce the server load by overwritting a the index.php, index.html script of the website with a one stating the server is ongoing a maintenance operations.
Temporary stopping the index page, often reduces the load in 10 seconds of time, so the second if case is not necessery at all. Sometimes, however this first "if" condition cannot decrease enough the load and the server load continues to stay too high, then the script second if comes to play and makes apache to be completely stopped via Apache init script do 2 secs delay and launch the apache server again.

The script also logs about, the load avarage encountered, while the server was overloaded and Apache webserver was restarted, so later I can check what time the server overload occured.
To make the script periodically run, I've scheduled the script to launch every 5 minutes as a cron job with the following cron:

# restart Apache if load is higher than 25
*/5 * * * * /usr/sbin/restart_apache_on_high_load.sh >/dev/null 2>&1

I have also another system which is running FreeBSD 7_2, which is having the same overload server problems as with the Linux host.
Copying the auto restart apache on high load script on FreeBSD didn't work out of the box. So I rewrote a little chunk of the script to make it running on the FreeBSD host. Hence, if you would like to auto restart Apache or any other service on FreeBSD server get /usr/sbin/restart_apache_on_high_load_freebsd.sh my script and set it on cron on your BSD.

This script is just a temporary work around, however as its obvious that the frequency of the high overload will be rising with time and we will need to buy new server hardware to solve permanently the issues, anyways, until this happens the script does a great job 🙂

I'm aware there is also alternative way to auto restart Apache webserver on high server loads through using monit utility for monitoring services on a Unix system. However as I didn't wanted to bother to run extra services in the background I decided to rather use the up presented script.

Interesting info to know is Apache module mod_overload exists – which can be used for checking load average. Using this module once load avarage is over a certain number apache can stop in its preforked processes current serving request, I've never tested it myself so I don't know how usable it is. As of time of writting it is in early stage version 0.2.2
If someone, have tried it and is happy with it on a busy hosting servers, please share with me if it is stable enough?