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  • Copy files from sub directories into one directory.

    - by Derek Organ
    Ok I have a bunch of files in this file structure format. /backup/daily/database1/database1-2011-01-01.sql /backup/daily/database1/database1-2011-01-02.sql /backup/daily/database1/database1-2011-01-03.sql /backup/daily/database1/database1-2011-01-04.sql /backup/daily/database1/database1-2011-01-05.sql /backup/daily/database1/database1-2011-01-06.sql /backup/daily/database1/database1-2011-01-07.sql /backup/daily/anotherdb/anotherdb-2011-01-01.sql /backup/daily/anotherdb/anotherdb-2011-01-02.sql /backup/daily/anotherdb/anotherdb-2011-01-03.sql /backup/daily/anotherdb/anotherdb-2011-01-04.sql /backup/daily/anotherdb/anotherdb-2011-01-05.sql /backup/daily/anotherdb/anotherdb-2011-01-06.sql /backup/daily/anotherdb/anotherdb-2011-01-07.sql /backup/daily/stuff/stuff-2011-01-01.sql /backup/daily/stuff/stuff-2011-01-02.sql /backup/daily/stuff/stuff-2011-01-03.sql /backup/daily/stuff/stuff-2011-01-04.sql /backup/daily/stuff/stuff-2011-01-05.sql /backup/daily/stuff/stuff-2011-01-06.sql /backup/daily/stuff/stuff-2011-01-07.sql And there are lots lots more. ultimately I want to import all the 2011-01-07.sql files into my mysql database. This works for one mysql -u root -ppassword < /backup/daily/database1/database1-2011-01-07.sql That will nicely restore that database from this backupfile. I want to run a process where it does this for all databases. So my plan is to first cp all 2011-01-07 sql files into a tmp dir e.g. cp /backup/daily/*/*2011-01-07*.sql /tmp/all The command above unfortunately isn't working I get an error: cp: cannot stat ..... No such file or directory So can you guys help me out with this. For bonus points if you can tell me how to do the next step which is import all databases in one command doing one at a time that would be great too. I really want to do these in two separate steps because I need to delete a few sql files manually from the tmp dir before I run the restore command. So I need: 1) command to copy all 2011-01-07 sql files to a tmp dir 2) command to import all those files in that dir into mysql I know its possible to do in one but for lots of reasons I really would prefer to do it in two steps.

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  • Will Vimperator always be this awesome?

    - by Martín Fixman
    About a week ago I started using Vim, and fell completely in love with it. However, today I installed the Vimperator extension on Firefox, and through there are some problems (all of which will be solved after using it until I get used to it), I found it great. However, I'm still in the "Holy fuck this is totally awesome" phase of software testing, and in some time will go back to the "I have this thing" phase. Just to be sure, will it be a good idea to use it regularly? I want to hear experiences about users and ex-users.

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  • Start multiple instances of Firefox

    - by Vi
    How can I have multiple independent instances of Mozilla Firefox 3.5 on the same X server, but started from different user accounts (consequently, different profiles)? Limited success was only with Xephyr :1, DISPLAY=:1 /usr/local/bin/firefox, but Xephyr has no Cygwin/X's "rootless" mode so it's not comfortable (see other question). The idea is to have one Firefox instance for various "Serious Business" things and the other for regular browsing with dozens of add-ons securely isolated.

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  • IPtables: DNAT not working

    - by GetFree
    In a CentOS server I have, I want to forward port 8080 to a third-party webserver. So I added this rule: iptables -t nat -A PREROUTING -p tcp --dport 8080 -j DNAT --to-destination thirdparty_server_ip:80 But it doesn't seem to work. In an effort to debug the process, I added these two LOG rules: iptables -t mangle -A PREROUTING -p tcp --src my_laptop_ip --dport ! 22 -j LOG --log-level warning --log-prefix "[_REQUEST_COMING_FROM_CLIENT_] " iptables -t nat -A POSTROUTING -p tcp --dst thirdparty_server_ip -j LOG --log-level warning --log-prefix "[_REQUEST_BEING_FORWARDED_] " (the --dport ! 22 part is there just to filter out the SSH traffic so that my log file doesn't get flooded) According to this page the mangle/PREROUTING chain is the first one to process incomming packets and the nat/POSTROUTING chain is the last one to process outgoing packets. And since the nat/PREROUTING chain comes in the middle of the other two, the three rules should do this: the rule in mangle/PREROUTING logs the incomming packets the rule in nat/PREROUTING modifies the packets (it changes the dest IP and port) the rule in nat/POSTROUTING logs the modified packets about to be forwarded Although the first rule does log incomming packets comming from my laptop, the third rule doesn't log the packets which are supposed to be modified by the second rule. It does log, however, packets that are produced in the server, hence I know the two LOG rules are working properly. Why are the packets not being forwarded, or at least why are they not being logged by the third rule? PS: there are no more rules than those three. All other chains in all tables are empty and with policy ACCEPT.

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  • Slight delay when switching modes in vim using tmux or screen

    - by Ton van den Heuvel
    Switching to and from insert mode in Vim is no longer instantaneous since I use tmux. After pressing Esc in insert mode, it takes a noticeable amount of time to actually get out of insert mode. After pressing Esc and any other key afterwards the switch is immediate, and the command for the key pressed after Esc is executed. Any idea what might cause this? The Vim configuration is not the problem as the delay does not occur when I run Vim outside tmux, so this is probably related to tmux somehow. I use gnome-terminal btw. Also worth noting, it seems I can not define key bindings in tmux for Esc, my plan was to bind Esc to: bind Escape send-keys ^[ Alas, it seems binding anything to Esc for tmux does not work. The same problem occurs in screen as well.

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  • Error while loading shared libraries - libwebsock

    - by kittyPL
    Im trying to setup libwebsock, simple C websocket library. I followed the installation procedure from INSTALL file, everything went fine. Im able to compile test program given in the examples. But when I want to run my executable, wild error appears: ./echo: error while loading shared libraries: libwebsock.so.1: cannot open shared object file: No such file or directory I checked /usr/local/lib twice, libwebsock.so.1 exists and is doing very well. I also tried copying the lib to the echo folder (so its placed next to binary), still same error. It's quite funny for me: shadowz@Ubu:~/WebSocket$ ls echo echo.c echo.cpp libwebsock.so.1 shadowz@Ubu:~/WebSocket$ ./echo ./echo: error while loading shared libraries: libwebsock.so.1: cannot open shared object file: No such file or directory Any suggestions? Im running out of ideas...

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  • syslog ip ranges to specific files using `rsyslog`

    - by Mike Pennington
    I have many Cisco / JunOS routers and switches that send logs to my Debian server, which uses rsyslogd. How can I configure rsyslogd to send these router / switch logs to a specific file, based on their source IP address? I do not want to pollute general system logs with these entries. For instance: all routers in Chicago (source ip block: 172.17.25.0/24) to only log to /var/log/net/chicago. all routers in Dallas (source ip block 172.17.27.0/24) to only log to /var/log/net/dallas. Finally, these logs should be rotated daily for up to 30 days and compressed. NOTE: I am answering my own question

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  • Apache security for multi-user development web server.

    - by mrmartinblue
    I've been searching and reading through documents all morning and understand that I need to use some combination of chown and probably 'jailing' to securely give programmers access to directories on my centos webserver. Here's the situation: I have an apache web server that has any number of virtual sites located in /var/www/site1 /var/www/site2 etc.. I have different developers that need full access both ssh and vsFTP to only the site they are working on. What is the best way to create and maintain security in this scenario. My thought would be to create a new user for each coder, jail that user to the website directory they are allowed to work in, add their user to a group and set the webroot's owner to that group. Any thoughts? Good, bad, ugly? Thanks!

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  • Making libmagic/file detect .docx files

    - by Jonatan Littke
    As seen elsewhere, docx, xlsx and pttx are ZIPs. When uploading them to my web application, file (via libmagic andpython-magic) detects them as being ZIP. I store the contents of the file as a blob in the database, but naturally I don't want to trust the user with what kind of file type this is. So I would like to trust file for and automatically generate a filename during download. I know one can modify /etc/magic but the format (magic(5)) is way too complicated for me. I found a bug report on the issue at Debian bugs but since it's from 2008 it doesn't seem to be fixed any time soon. I guess my only other alternative is to indeed trust the user (but still store the contents as a blob) and only check the file extension based on the file name. This way I can disallow some extensions and allow others. And when the user re-downloads his file, he can have it in whatever way he uploaded it. But this solution is insecure if the file is shared with others, since you can simply rename the file to allow uploading it. Any ideas? Lastly, I found a list of magic numbers for docx etc, but I'm unable to convert these into the magic(5) format.

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  • X Session from Mac

    - by tekknolagi
    How can I log into an X server from Mac OS X? I know that ssh -X username@host will log me in and I will have the capability to run X applications. On Cygwin/X you can log in and have a whole X session from your computer... and it will look something like this: How can I replicate this? Using this batch script: @echo off SET DISPLAY=127.0.0.1:0.0 SET REMOTE_HOST=%1 IF "%REMOTE_HOST%" == "" SET REMOTE_HOST=10.0.0.1 SET CYGWIN_ROOT=\cygwin SET RUN=%CYGWIN_ROOT%\bin\run -p /usr/bin SET PATH=.;%CYGWIN_ROOT%\bin;%PATH% SET XAPPLRESDIR= SET XCMSDB= SET XKEYSYMDB= SET XNLSPATH= if not exist %CYGWIN_ROOT%\tmp\.X11-unix\X0 goto CLEANUP-FINISH attrib -s %CYGWIN_ROOT%\tmp\.X11-unix\X0 del %CYGWIN_ROOT%\tmp\.X11-unix\X0 :CLEANUP-FINISH if exist %CYGWIN_ROOT%\tmp\.X11-unix rmdir %CYGWIN_ROOT%\tmp\.X11-unix if "%OS%" == "Windows_NT" goto OS_NT echo startxdmcp.bat - Starting on Windows 95/98/Me goto STARTUP :OS_NT REM Windows NT/2000/XP echo startxdmcp.bat - Starting on Windows NT/2000/XP :STARTUP %RUN% XWin -query tekknolagi.dyndns.org -clipboard -lesspointer -scrollbars -screen 0 1050x1655@2 -screen 1 1680x985@1

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  • GNU Screen: Combining split-regions and full-screen sessions

    - by scrrr
    Let's say I have three sessions: 0, 1 and 2 I'm on session 0 and I press CTRL-A S to split the screen. Then I select session 1 for the bottom split region, while 0 is in the upper. Can I switch to session 2 and have it display in full-screen while 0 and 1 remain split? If I CTRL-A n to other sessions in a split screen it only changes the split-region. I want some sessions to be full-screen though. Is that possible?

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  • How do I keep gnome-terminal width from changing when I use "reset"

    - by monguin
    When I use reset in gnome-terminal (bash, Ubuntu 9.04), the window width resets, though the height does not. On my personal computer, this does not happen in xterm or konsole. On my work computer (RHEL 5 or maybe 4), this does not happen in any terminals, including gnome-terminal. I assume there is some environment variable that controls this, but my .bashrc at work is hundreds of lines long, so I'm not sure where to look in there.

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  • Ubuntu 10.04 freezing and Ctrl + Alt + Backspace does nothing but music keeps playing

    - by Bryce Thomas
    I'm having intermittent problems where the screen will freeze in Ubuntu. I've tried using Ctrl + Alt + Backspace to restart the X-server, though this does nothing. When the freeze occurs, there's a small square of black dashes around the mouse pointer - maybe 1 inch in size. These dashes look a lot like a 2d barcode. The rest of the screen looks normal, but I can't move the mouse and none of the keyboard shortcuts work to do anything. However, music that I begin playing before the freeze continues to play, which seems to indicate it hasn't stalled up completely. I've noticed a similar freezing problem when I'm using Windows 7. That is, I see the same barcode like dashes around the mouse pointer when it freezes up. So I'm guessing it's either a driver or hardware problem. I thought if it was a hardware problem though, the whole computer might stop working (i.e. music would stop playing)? The video card I am using is an Nvidia, and I believe it's in the 7600 range. In Ubuntu I have the drivers for the card set to the latest available (proprietary). Ideally I'd like to be able to continue using the proprietary drivers. Is there any known issues with the drivers for this model graphics card, or has anyone experienced the same problem and knows how to fix it?

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  • Enabling vim file type plugin

    - by person
    In my .vimrc file I have this line.. filetype plugin indent on and then in ~/.vim/ftplugin/py.vimrc (I've also tried saving it as a .vim file) I've copied and pasted a vimrc file for python standards and have enabled all of the suggested settings in it (uncommented them). When I create and open a test.py file and do something like a tab, it goes 8 spaces instead of 4. Not sure what's going wrong.

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  • Run a script after killing lxsession (xorg)

    - by user284194
    I am trying to run a program automatically within a bash script after killing the LXDE session. My script consists of: #!/bin/sh pkill lxsession; sh /home/pi/RetroPie/EmulationStation/emulationstation My aim is to log out of the LXDE session and run EmulationStation on my Raspberry Pi with a bash script. I'm using pkill lxsession; to bypass lxsession's logout confirmation dialog. As it stands, this script just gets me to the command line from a working LXDE desktop. Thanks for reading.

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  • Should tripwire be entering /proc?

    - by dsadinoff
    When initializing the db with tripwire --init it spat out a bunch of errors pertaining to /proc: ### Warning: File system error. ### Filename: /proc/16982/fd/4 ### No such file or directory ### Continuing... ### Warning: File system error. ### Filename: /proc/16982/fdinfo/4 ### No such file or directory ### Continuing... ### Warning: File system error. ### Filename: /proc/16982/task/16982/fd/4 ### No such file or directory ### Continuing... ### Warning: File system error. ### Filename: /proc/16982/task/16982/fdinfo/4 ### No such file or directory ### Continuing... ### Warning: Duplicate object encountered. ### /proc/sys/net/ipv6/neigh This feels like noise. The twpol.txt file has the following clause: # # Critical devices # ( rulename = "Devices & Kernel information", severity = $(SIG_HI), ) { /dev -> $(Device) ; /proc -> $(Device) ; } Which, if I understand it right, is going to cause tripwire to care deeply about the entire contents of /proc. Shouldn't it just care about the static parts of /proc like the drivers and such, and not the per-pid stuff? Why does it ship like this?

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  • How to configure autofs5 timeout on per-filesystem basis?

    - by Norman Ramsey
    Because of a show-stopping bug in Debian autofs 4, I just upgraded to autofs5. It is not honoring the timeout option in my auto.master file: /var/autofs/removable /etc/auto.removable --timeout=2 I use this map for thumb drives and so on; I don't want a general default timeout of 2 seconds. I did some digging and although the --timeout option worked in autofs 4, and it appears in some examples on the Web, it is not actually sanctioned (or even mentioned) in the documentation for the auto.master file. So I don't feel I can report the problem as a bug. How can I get autofs5 to timeout after 2 seconds only on designated filesystems? Update: I am using a Debian-packaged autofs5, version 5.0.4-3.2.

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  • Sendmail doesn't work with iptables, even though smtp and dns are allowed

    - by tom
    I have sendmail installed on Ubuntu 10.04 solely for the use of the php mail() function. This works fine unless iptables is running (I've been using sendmail [email protected] to test this). I think that I have allowed SMTP and DNS (the script I am using to test iptables rules is below, in my version are the actual IPs of my hosts nameservers), but to no avail! iptables --flush iptables -A INPUT -p tcp --dport 22 -j ACCEPT iptables -A INPUT -p tcp --dport 80 -j ACCEPT iptables -A INPUT -p tcp --dport 443 -j ACCEPT # Postgres iptables -A INPUT -p tcp --dport 5432 -j ACCEPT # Webmin iptables -A INPUT -p tcp --dport 10000 -j ACCEPT # Ping iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT iptables -A OUTPUT -p icmp --icmp-type echo-reply -j ACCEPT # sendmail iptables -A INPUT -p tcp --dport 25 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A OUTPUT -p tcp --sport 25 -m state --state ESTABLISHED -j ACCEPT # DNS iptables -A INPUT -p udp --sport 53 -s <nameserver1> -j ACCEPT iptables -A INPUT -p udp --sport 53 -s <nameserver2> -j ACCEPT iptables -A INPUT -p tcp --sport 53 -s <nameserver1> -j ACCEPT iptables -A INPUT -p tcp --sport 53 -s <nameserver2> -j ACCEPT iptables -A OUTPUT -p udp --dport 53 -d <nameserver1> -j ACCEPT iptables -A OUTPUT -p udp --dport 53 -d <nameserver2> -j ACCEPT iptables -A OUTPUT -p tcp --dport 53 -d <nameserver1> -j ACCEPT iptables -A OUTPUT -p tcp --dport 53 -d <nameserver2> -j ACCEPT iptables -A INPUT -j DROP # Add loopback iptables -I INPUT 1 -i lo -j ACCEPT

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  • Perform action based on load avg

    - by sfx
    I'm running some web applications on an debian server and have to struggle with ddos attacks sometimes. It's eating up all my resources and I can't ssh anymore into the server. An idea was to drop all connections if the load avg is too high, so there are still resources for me and accept new connections if the load avg is low enough. Since this has to work under heavy load I'm afraid a cronjob wouldn't be fast enough or take too much resources. tl;dr: Is there a way to configure the behavior if the load avg is above a specific threshold?

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  • Running Mathimatica-5 remotely

    - by oxinabox.ucc.asn.au
    Ok, I have Mathmatica 5 - a powerful CAS. I have a cheap netbook, wich not olny is too slow to run mathmatica on, I doubt it has the harddrive space. I do however have remote access to a number of very powerful computers, (most of wich run variose linuxes, but one of which is windows server 2008) Mostly over SSH but other protocols can be arraged for some, i'm sure. (I might even be able to remote desktop the windows server 2008) So I'ld like to install Mathmatica onto one of these machine and then run it remotely. Either from the command line via putty or via some other method. I glanced through the mathmatical documentaion and read soemthing about using some MathLink program, wich linkes the front end istalled on my computer to a remote kernal. Anyone have any expirience with this? I'm not sure if this belongs here or in SuperUser.

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  • vlans on openvz, centos 6

    - by arheops
    i have centos 6 with openvz installed on it, switch with vlan support. I need following setup: 1) eth0 on openvz have be tagged multiple vlans. 2) each virtualhost have to be in single vlan. yes,i already read wiki on openvz, but it is just not work. I have on main server interface eth0.108 and able ping address on that interface(using nootbook on untagged port vlan 108), but i not able ping address inside container. Main node: [root@box1 conf]# ifconfig eth0 Link encap:Ethernet HWaddr D0:67:E5:F4:11:60 inet6 addr: fe80::d267:e5ff:fef4:1160/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:506 errors:0 dropped:0 overruns:0 frame:0 TX packets:25 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:68939 (67.3 KiB) TX bytes:1780 (1.7 KiB) Interrupt:16 Memory:c0000000-c0012800 eth0.108 Link encap:Ethernet HWaddr D0:67:E5:F4:11:60 inet addr:10.11.108.3 Bcast:10.11.111.255 Mask:255.255.252.0 inet6 addr: fe80::d267:e5ff:fef4:1160/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:238 errors:0 dropped:0 overruns:0 frame:0 TX packets:19 errors:0 dropped:12 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:25890 (25.2 KiB) TX bytes:926 (926.0 b) eth1 Link encap:Ethernet HWaddr D0:67:E5:F4:11:61 inet addr:192.168.23.233 Bcast:192.168.23.255 Mask:255.255.255.0 inet6 addr: fe80::d267:e5ff:fef4:1161/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:1967 errors:0 dropped:0 overruns:0 frame:0 TX packets:356 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:365298 (356.7 KiB) TX bytes:115007 (112.3 KiB) Interrupt:17 Memory:c2000000-c2012800 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:7 errors:0 dropped:0 overruns:0 frame:0 TX packets:7 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:784 (784.0 b) TX bytes:784 (784.0 b) venet0 Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00 inet6 addr: fe80::1/128 Scope:Link UP BROADCAST POINTOPOINT RUNNING NOARP MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:3 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:0 (0.0 b) TX bytes:0 (0.0 b) veth108.0 Link encap:Ethernet HWaddr 00:18:51:DA:94:D5 inet6 addr: fe80::218:51ff:feda:94d5/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:639 errors:0 dropped:0 overruns:0 frame:0 TX packets:5 errors:0 dropped:1 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:17996 (17.5 KiB) TX bytes:308 (308.0 b) virtual node [root@pbx108 /]# ifconfig eth0.108 Link encap:Ethernet HWaddr 00:18:51:CA:B5:C5 inet addr:10.11.108.1 Bcast:10.11.111.255 Mask:255.255.252.0 inet6 addr: fe80::218:51ff:feca:b5c5/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:5 errors:0 dropped:0 overruns:0 frame:0 TX packets:685 errors:0 dropped:2 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:308 (308.0 b) TX bytes:19284 (18.8 KiB) lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:683 errors:0 dropped:0 overruns:0 frame:0 TX packets:683 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:76288 (74.5 KiB) TX bytes:76288 (74.5 KiB) /etc/vz/conf/108.conf # RAM PHYSPAGES="0:4000M" # Swap SWAPPAGES="0:512M" # Disk quota parameters (in form of softlimit:hardlimit) DISKSPACE="200G:200G" DISKINODES="20000000:22000000" QUOTATIME="0" # CPU fair scheduler parameter CPUUNITS="4000" VE_ROOT="/vz/root/$VEID" VE_PRIVATE="/vz/private/$VEID" OSTEMPLATE="centos-6-x86_64" ORIGIN_SAMPLE="vswap-256m" NETIF="ifname=eth0.108,mac=00:18:51:CA:B5:C5,host_ifname=veth108.0,host_mac=00:18:51:DA:94:D5" NAMESERVER="8.8.8.8" HOSTNAME="pbx108.localhost" IP_ADDRESS=""

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  • How to repair unbootable Fedora install

    - by Cerin
    How do you repair/reinstall Fedora without deleting any existing partitions or data? I was attempting to upgrade some old Fedora 13 servers to 17, following the instructions in the wiki. After the 14-15 upgrade, rebooting resulted in the output: Dropping to debug shell. sh: can't access tty; job control turned off dracut:/# Running dmesg also shows: dracut Warning: No root device "block:/dev/mapper/VolGroup-lv_root" found Googling shows this error is typically related to some weird RAID issues, but my server is a virtual machine not using any RAID. Using a rescue CD, I can chroot /mnt/sysimage, and all packages and data still seems to be there. How do I make the system bootable again?

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  • Can enabling a RAID controller's writeback cache harm overall performance?

    - by Nathan O'Sullivan
    I have an 8 drive RAID 10 setup connected to an Adaptec 5805Z, running Centos 5.5 and deadline scheduler. A basic dd read test shows 400mb/sec, and a basic dd write test shows about the same. When I run the two simultaneously, I see the read speed drop to ~5mb/sec while the write speed stays at more or less the same 400mb/sec. The output of iostat -x as you would expect, shows that very few read transactions are being executed while the disk is bombarded with writes. If i turn the controller's writeback cache off, I dont see a 50:50 split but I do see a marked improvement, somewhere around 100mb/s reads and 300mb/s writes. I've also found if I lower the nr_requests setting on the drive's queue (somewhere around 8 seems optimal) I can end up with 150mb/sec reads and 150mb/sec writes; ie. a reduction in total throughput but certainly more suitable for my workload. Is this a real phenomenon? Or is my synthetic test too simplistic? The reason this could happen seems clear enough, when the scheduler switches from reads to writes, it can run heaps of write requests because they all just land in the controllers cache but must be carried out at some point. I would guess the actual disk writes are occuring when the scheduler starts trying to perform reads again, resulting in very few read requests being executed. This seems a reasonable explanation, but it also seems like a massive drawback to using writeback cache on an system with non-trivial write loads. I've been searching for discussions around this all afternoon and found nothing. What am I missing?

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