The Basics The btrfs file system is designed to meet the expanding scalability requirements of large storage subsystems. As the btrfs file system uses B-trees in its implementation, its name derives from the name of those data structures, although it is not a true acronym. A B-tree is a tree-like data structure that enables file systems and databases to efficiently access and update large blocks of data no matter how large the tree grows. The btrfs file system provides the following … [Read more...] about How to Create and Query a BTRFS File System
How to configure CentOS/RHEL 6 system to not used last 3 passwords used
Question: How to change the password policy on a system so that a user cannot choose any of the last 3 passwords previously used? 1. Make sure that the file /etc/security/opasswd has the default permission (600). # ls -l /etc/security/opasswd -rw------- 1 root root 116 Sep 8 14:52 /etc/security/opasswd 2. Ensure you backup the file /etc/pam.d/system-auth: # cp /etc/pam.d/system-auth /etc/pam.d/system-auth.old 3. Modify the file /etc/pam.d/system-auth in vi editor and change the line … [Read more...] about How to configure CentOS/RHEL 6 system to not used last 3 passwords used
How to Customize Linux Password Expiration and Complexity Requirements
1. Password Expiration To set the default password expiration when creating new accounts on CentOS/RHEL, edit the /etc/login.defs file. PASS_MAX_DAYS 100 Which means the maximum number of days a password may be used. Please check man page of login.defs for more options that can be set in /etc/login.defs.: $ man login.defs 2. Password Complexity The PAM module pam_cracklib can be used to force password complexity requirements. The complexity can be specified with the following … [Read more...] about How to Customize Linux Password Expiration and Complexity Requirements
Solaris Interview Questions – Inodes and the Filesystem Troubleshooting
1. Why 'du' and 'df' reports different values? - df describes disk usage. - du adds up the size all the files on a filesystem. They can report wildly different figures. One reason: Processes write to files. files can be deleted while processes are writing to files. So a file may be gone, but its inode may not be freed up. For example: # cat > bogus testing testing ^Z Stopped (user) # ls -i bogus 2921 bogus # fuser bogus bogus: 14947o # rm bogus # ls -i bogus bogus: No … [Read more...] about Solaris Interview Questions – Inodes and the Filesystem Troubleshooting
How To Masquerade Sender Address In Sendmail in CentOS/RHEL
You would like to configure your sendmail server to masquerade the sender address to no-reply@target-domain.net when sending emails from it. The Solution Normally to masquerade a domain, the "MASQUERADE_AS" and "MASQUERADE_DOMAIN" is used in /etc/mail/sendmail.mc. But that will leave the user unchanged. With sendmail, you can change both the domain and username on a case-by-case basis using the genericstable feature. 1. make sure all the "MASQUERADE" items are commented out in … [Read more...] about How To Masquerade Sender Address In Sendmail in CentOS/RHEL
How To Send Mails To an External User With Mailx on Linux
Question: How to send emails to external users with mailx running on CentOS/RHEL system? The following command shows how to send a mail to an external user using mailx: # echo "this is the body of the email" | mailx -vvv -s "test mail" -r "From" -S smtp="your-smtp" someone@address Here, -vvv = Verbosity. -s = Specifies the subject. -r = Email sent from. -S = Specifies the smtp server. To be able to send mails out of the internal network using … [Read more...] about How To Send Mails To an External User With Mailx on Linux
How to Replace a Failed Btrfs Device
The Problem Originally the btrfs file system is build on multiple disks. # btrfs filesystem show Label: 'MYBTRFS' uuid: 09138ad8-eb2f-4c7c-aef7-90482a67c45c Total devices 3 FS bytes used 768.00KiB devid 1 size 5.00GiB used 1.52GiB path /dev/xvdb devid 2 size 5.00GiB used 520.00MiB path /dev/xvdc devid 3 size 5.00GiB used 1.51GiB path /dev/xvdd For some reason one disk is faulty and needs to be replaced. # btrfs filesystem show Label: 'MYBTRFS' uuid: … [Read more...] about How to Replace a Failed Btrfs Device
How to Check Btrfs FileSystem Usage and Perform Balancing
In Btrfs file system there are chunks that are allocated for regular data, metadata. There are also chunks that are allocated to store file system information and also about where other chunks are located on the physical disk. Those chunks can only store the data. Very occasionally, a chunk type may fill up and start to run out of space. However, there may (and often is) still space available in other chunks. Therefore, some space needs to be freed up on it. There is a balancing feature … [Read more...] about How to Check Btrfs FileSystem Usage and Perform Balancing
How to Re-Balance BTRFS to Free Disk Space
The Problem The BTRFS filesystem has filled on the server, and thus some space needs to be freed up on it. There is a re-balancing feature available in btrfs. The primary purpose of the re-balance feature is to spread block groups across all devices so they match constraints defined by the respective profiles and this is done when btrfs need re-balancing to fix issues with chunks being unbalanced. The Solution It is common for a btrfs device to fill up due to the fact that it does not … [Read more...] about How to Re-Balance BTRFS to Free Disk Space
BTRFS: too many missing devices, writeable mount is not allowed
The Problem Assembling a BTRFS device during boot fails with error BTRFS: too many missing devices, writeable mount is not allowed The disks have been mistakenly deleted The Solution The missing disks in the raid array do not allow the filesystem to be mounted. Below are the logs from dmesg: [ 44.706956] btrfs: disk space caching is enabled [ 44.708006] btrfs: failed to read the system array on sdd1 [ 44.721272] btrfs: open_ctree failed [ 45.437152] nr_pdflush_threads exported in … [Read more...] about BTRFS: too many missing devices, writeable mount is not allowed