Snowcrash's questions

Icinga notified us of the presence of a /var/run/reboot-required file on various servers. i.e. we were getting a

SERVICE STATUS: CRITICAL file or directory /var/run/reboot-required exists message in Icinga.

We duly did a reboot only to find a couple of days later the message had cropped up again.

I don't recall having to reboot servers so often. I'm starting to think these are just spurious messages and we should simply ignore them.

Is there any harm in that?

This page:

http://nginx.org/en/docs/http/ngx_http_core_module.html#resolver

gives this example:

resolver 127.0.0.1 [::1]:5353;

What does the [::1] and :5353 mean?

Registered hosts blurb say:

Register public hosts on your domain by IP address so they can be found 
without first resolving your domain in the DNS. Entries here are commonly 
called "glue records" and are needed when a domain's nameservers serve on one 
of its subdomains.

but Custom Records say:

Resource records define how your domain behaves. Common uses include pointing 
your domain at your web server or configuring email delivery for your domain.

If I want to register a public web host by IP address which one should I use?

E.g. www.mydomain.com -> 1.1.1.1

What, specifically, are the 2/2 checks that come under the Status Checks column in the AWS EC2 console?

I'm creating an ELB and it says:

Please select at least two Subnets in different Availability Zones to provide higher availability for your load balancer.

However, it doesn't seem to require this.

E.g. if I add just one subnet then I can go ahead and create the LB.

Is this just an optional message?

AWS' Application Load Balancer has the feature to Route to multiple ports on a single instance.

Can anyone name a few use cases when you might use this feature?

How do you remove a PV from a VG?

Here's what I've tried:

# vgreduce vg_iscsi /dev/sdb1
Physical volume "/dev/sdb1" still in use

# lvs
LV         VG       Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  root       centos   -wi-ao----   6.67g                                                    
  swap       centos   -wi-ao---- 820.00m                                                    
  lvm02      vg2      -wi-a----- 800.00m                                                    
  lv_iscsi_1 vg_iscsi -wi-ao----  52.00m  

Then

# lvremove -f vg_iscsi/lv_iscsi_1
Logical volume vg_iscsi/lv_iscsi_1 in use.

Using fuser

# fuser -kuc /dev/vg_iscsi/lv_iscsi_1
/dev/dm-3:            1558(root)

I tried to kill -9 this but the process seemed to have disappeared by the time I issued the kill signal.

However, now I get this:

# vgreduce vg_iscsi /dev/sdb1
Can't remove final physical volume "/dev/sdb1" from volume group "vg_iscsi"

Any ideas?

I've made a partition on CentOS7 and rebooted.

However when I now issue mkswap on that partition I get a busy message:

mkswap /dev/sdb1
/dev/sdb1: Device or resource busy

What's the problem?

====

[root@centos1 ~]# df -h
Filesystem               Size  Used Avail Use% Mounted on
/dev/mapper/centos-root  6.7G  5.1G  1.6G  77% /
devtmpfs                 356M     0  356M   0% /dev
tmpfs                    371M   84K  371M   1% /dev/shm
tmpfs                    371M  5.4M  366M   2% /run
tmpfs                    371M     0  371M   0% /sys/fs/cgroup
/dev/sda1                497M  209M  288M  43% /boot
tmpfs                     75M   20K   75M   1% /run/user/42
tmpfs                     75M     0   75M   0% /run/user/1000

[root@centos1 ~]# mount
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime,seclabel)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
devtmpfs on /dev type devtmpfs (rw,nosuid,seclabel,size=364304k,nr_inodes=91076,mode=755)
securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev,seclabel)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,seclabel,gid=5,mode=620,ptmxmode=000)
tmpfs on /run type tmpfs (rw,nosuid,nodev,seclabel,mode=755)
tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,seclabel,mode=755)
cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/usr/lib/systemd/systemd-cgroups-agent,name=systemd)
pstore on /sys/fs/pstore type pstore (rw,nosuid,nodev,noexec,relatime)
cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,hugetlb)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpuacct,cpu)
cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)
cgroup on /sys/fs/cgroup/net_cls type cgroup (rw,nosuid,nodev,noexec,relatime,net_cls)
cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer)
cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio)
cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,perf_event)
configfs on /sys/kernel/config type configfs (rw,relatime)
/dev/mapper/centos-root on / type xfs (rw,relatime,seclabel,attr2,inode64,noquota)
selinuxfs on /sys/fs/selinux type selinuxfs (rw,relatime)
systemd-1 on /proc/sys/fs/binfmt_misc type autofs (rw,relatime,fd=25,pgrp=1,timeout=300,minproto=5,maxproto=5,direct)
hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,seclabel)
debugfs on /sys/kernel/debug type debugfs (rw,relatime)
mqueue on /dev/mqueue type mqueue (rw,relatime,seclabel)
sunrpc on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw,relatime)
nfsd on /proc/fs/nfsd type nfsd (rw,relatime)
/dev/sda1 on /boot type xfs (rw,relatime,seclabel,attr2,inode64,noquota)
tmpfs on /run/user/42 type tmpfs (rw,nosuid,nodev,relatime,seclabel,size=75880k,mode=700,uid=42,gid=42)
tmpfs on /run/user/1000 type tmpfs (rw,nosuid,nodev,relatime,seclabel,size=75880k,mode=700,uid=1000,gid=1000)

And, fwiw:

[root@centos1 ~]# fdisk /dev/sdb
Welcome to fdisk (util-linux 2.23.2).

Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.


Command (m for help): p

Disk /dev/sdb: 8589 MB, 8589934592 bytes, 16777216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0xe82746a5

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1            2048     4196351     2097152   83  Linux

====

# pvs
PV         VG       Fmt  Attr PSize  PFree 
/dev/sda2  centos   lvm2 a--   7.51g 40.00m
/dev/sdb1  vg_iscsi lvm2 a--  96.00m 44.00m

I'm wanting to add a Swap partition to a CentOS 7 machine (running in VirtualBox). However, when I try and add it I get this error:

partx -a /dev/sdd1
partx: /dev/sdd: error adding partition 1

Here's the output of fdisk /dev/sdd

Command (m for help): p

Disk /dev/sdd: 8589 MB, 8589934592 bytes, 16777216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x1b359154

   Device Boot      Start         End      Blocks   Id  System
/dev/sdd1            2048     4196351     2097152   83  Linux

Any suggestions?

I'm trying to list services on my CentOS image running in Docker using

systemctl list-units  

but I get this error message:

Failed to get D-Bus connection: Operation not permitted

Any suggestions what the problem might be?

I'm launching an EC2 instance via a CloudFormation template, however, the specified instance t2.micro requires a VPC.

How do I specify a VPC in the CloudFormation template?

Here's my template:

{
    "Description" : "Single Instance",

    "Resources" : {
        "EC2Instance" : {
            "Type" : "AWS::EC2::Instance",
            "Properties" : {
                "ImageId" : "ami-b73b63a0",
                "InstanceType" : "t2.micro",
                "KeyName" : "my-key",
                "Tags" : [
                    {
                        "Key" : "Name",
                        "Value" : "test"
                    }
                  ]
            }
        }
    }
}

How do you install the LVM GUI on CentOS7?

I've tried:

yum install system-config-lvm
Loaded plugins: fastestmirror, langpacks
Loading mirror speeds from cached hostfile
 * base: centos.mirroring.pulsant.co.uk
 * extras: centos.mirroring.pulsant.co.uk
 * updates: centos.mirroring.pulsant.co.uk
No package system-config-lvm available.
Error: Nothing to do

and Applications > System Tools > Application Installer which says lvm cannot be found.

Any suggestions?

I installed "Development Tools" with yum (`yum group install "Development Tools") and wanted to check it's installed. However, if I re-run the group install command I get:

Maybe run: yum groups mark install (see man yum)
No packages in any requested group available to install or update.

so I assume the group has installed. However is there a way to check? E.g. listing the packages so I can tick a few off.

Is there a neater way to list hard drives with fdisk?

If I run fdisk -l I get the Disk, Units, Sector size, I/O size, etc - all distracting stuff.

I can run fdisk -l | grep "Disk /" but this seems a bit of a hack. Is there a better way?

====

UPDATE

I thought I'd made myself perfectly clear. I'd given examples of what I didn't want and what I did want in the subsequent two lines of my question.

By neater I mean just listing the hard drives (i.e. Disk /dev/xvdb - see line 3 of my question) without all distracting stuff like Disk, Units, Sector size, I/O size (see line two of my question). i.e. not this:

8589 MB, 8589934592 bytes, 16777216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

I feel like I'm repeating myself.

I'm running this command:

find . -type f -delete

however Ubuntu keeps killing the process. Here's the output from /var/log/syslog:

Mar 11 19:54:14 hostname kernel: Killed process 29402 (find) total-vm:574220kB, anon-rss:513436kB, file-rss:580kB

Has this process exceeded some Ubuntu / Linux limit? And, if so, is there a better method than offered here https://stackoverflow.com/a/10077637/343204?

I'm seeing GET requests like:

"GET /assets/)!==t&&(e.html(e.data( HTTP/1.1" 200 3538 "-" "Java/1.7.0_25"

in my logs for a box hosting a Rails app. Looks suspiciously like someone probing the app but can't be sure.

Any suggestions?