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Course Intermediate

System Administration

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01

Process Management

The sampling-interval dependence of listing and searching processes, job control, signals turning into requests, the visible effect of the priority value, and the cause that resource limits hide.

  1. 01 Listing and Searching Processes On a synthetic server of twenty-four processes, 14 are actually heavy: once the sampling interval rises to 15 seconds the tool misses 3 of them, and at 30 and 60 seconds it misses 5; a single glance gives 5–7 false diagnoses, the lifetime average gives 7, and selecting by name touches 9 processes wrongly in the best case.
  2. 02 Foreground and Background Jobs When a session closes, all 13 of the shell's `&`-backgrounded jobs die, 9 of the disowned ones die, while nohup and a separate session keep all 13 alive; `jobs` shows 13 lines while the number of jobs actually progressing is 8.
  3. 03 Signals When a termination request is sent to seven heavy processes, 2 accept it and 5 ignore it; with the forced signal, 7/7 finish, but the cleanup of 2 processes is cut short. Because the command returns 0 in every case, the diagnosis given without waiting is wrong for all seven targets.
  4. 04 Process Priorities When the priority value of the process doing the most work is pulled from the lowest to the highest, its share drops from 41,878 units to 0, but total work goes from 237,906 to 238,244 and load goes from 3.97 to 3.97: 94 percent of the freed capacity goes to other heavy processes, 0 percent to light ones.
  5. 05 Resource Limits Eight distinct failure causes collapse into three error messages and one silence: 13 of 60 events produce no message at all, a diagnosis based on the message alone is wrong 21 times, one based on the limit reading alone is wrong 29 times, and the one combining both is wrong 13 times.

02

Services and Boot

The unit and dependency model, the gap between apparent status and being healthy, the failure a restart policy hides, the boot chain, and bootloader parameters.

  1. 01 The Service Manager Model The unit, target, and dependency concepts are established; the diagnosis given by the unit list is compared against the diagnosis given by the dependency graph at four levels of detail, and wrong diagnoses are counted.
  2. 02 Service Status and Control Starting, stopping, and enabling are established as separate axes; the status word of a unit stuck in a failure loop is compared against its real state, and how many of sixty crashes are seen at which polling interval is counted.
  3. 03 Defining a New Service The unit file's sections and fields are established; what three restart policies hide across three exit types is counted over nine combinations, and how far the start limit brings the hidden time down is swept.
  4. 04 The Boot Process The handoff chain from firmware to the first process and targets is established; how many units restarting one unit stops and starts, and how many units a unit failing at boot never starts at all, are counted.
  5. 05 Bootloaders The parts of bootloader configuration, kernel selection, and boot parameters are established; how many of twenty-four parameter changes actually do their job is counted with three verification methods, and the safe way to test irreversible operations is shown.

03

Logs and Diagnostics

System and service logs, the authentication trail, the evidence rotation deletes, load average's lag, and separating bottleneck types.

  1. 01 Log Architecture The distinction between the system log and service logs: where a record comes from, what level it is stamped with, and how many times a diagnosis looking at a single unit file turns out wrong.
  2. 02 Authentication Logs Tracing login, privilege request, and session-close events: the distinction between authentication and authorization, how many lines text-matching filters get wrong, and why an attack's most important line is the successful one, not a failed one.
  3. 03 Log Filtering and Rotation Measuring level, unit, and time filters together with the rotation policy: while 4000 lines have 86 errors, a 1000-line file keeps 19 and a 200-line file keeps 5, and how many of six investigation questions go unanswered is counted.
  4. 04 System Health Metrics What load average, memory, and disk indicators say and do not say: while the instantaneous load is 7.36, the sixty-second average stays at 6.22, and the averaged indicator never shows the real load that crosses the threshold for 54 seconds.
  5. 05 Tracing Performance Problems Separating CPU, I/O, and memory bottlenecks: a view producing four times as many lines is still wrong on 12 of 18 events, a three-line view drops to 2, and the reading order of the same three metrics removes the remaining two misses.

04

Storage

Block devices and partitions, file system families, inode exhaustion, mount options, logical volumes and snapshots, swap space, and disk expansion.

  1. 01 Block Devices and Partitions A partition list is not a map: while the device shows 3,702,784 free sectors, the largest contiguous gap is 2,881,536 sectors, and six of the twelve placement decisions drawn from the list turn out wrong.
  2. 02 File Systems A directory carrying the same files costs 449,985,000, 125,312, and 59,800 comparisons across three internal layouts; in a design carrying no checksum, all 54 of the reads that touch a corrupted block return without error.
  3. 03 Inodes and Metadata In the same file system, while block usage is 0.0125, inode usage is 1.0000 and a write fails: the disk is one percent full and the system is full. A file whose name is deleted but whose handle stays open hides 120,000 blocks from du's output.
  4. 04 Mounting and Persistent Mounts A persistent mount entry written to a device name mounts the correct file system in only thirteen of sixty boots and silently mounts the wrong one in twenty-seven; a mount option that does not update access time makes a cleanup job wrongly delete 787 files.
  5. 05 Logical Volume Management Growing is two steps, and seven of twenty-one readings point to the wrong link in the chain; a snapshot with 512 blocks allocated fills at the 413th second, and monitoring that looks once every sixty seconds sees a number sitting at 256 blocks until then.
  6. 06 Swap Space Pressure lasts 84 seconds, but the used-swap indicator raises a false alarm for 272 seconds, free memory for 451; as the swap area grows, the closability diagnosis rises from 44 wrong to 354 wrong.
  7. 07 Adding and Expanding a Disk The chain for joining a new device to the system is nine steps, and thirteen of forty-five readings say the work is done when it is not; a persistent entry written to a device name mounts the correct file system in only ten of forty boots.

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