vault backup: 2025-05-05 16:12:23

.obsidian/workspace.json

@@ -198,26 +198,27 @@
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     "Operating Systems/Memory Management.md",
-    "Operating Systems/assets/Pasted image 20250505042548.png",
-    "Operating Systems/Virtual Memory.md",
+    "Operating Systems/Overview.md",
     "Operating Systems/Scheduling.md",
     "Operating Systems/Processes and Threads.md",
+    "Operating Systems/Virtual Memory.md",
+    "Operating Systems/assets/Pasted image 20250505155546.png",
+    "Operating Systems/File Systems Management.md",
+    "Operating Systems/assets/Pasted image 20250505160518.png",
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Operating Systems/File Systems Management.md

@@ -29,7 +29,17 @@ Files are mapped by the OS onto physical nonvolatile devices
 - Size
 - Protection (permissions)
 - Datetime and user id
+All of these are stored in **i-nodes**.
 
+
+### INodes
+- Size in biytes
+- Access permissions
+- Type
+- Creation and last access datetime
+- Owner ID
+- Group ID
+- Hard link count
 ### Logical Definition
 - Named collection of related information
 - Files may have free form (text files) or can be rigidly formatted[^1]
@@ -92,6 +102,12 @@ Users have different directories. In Linux -> `/home/user` is separate, allowing
 ### Acyclic-Graph
 Have shared subdirectories and files. Symlinks achieve this.
 
+### Structure
+In Linux, it is a table (a file) which stores:
+- File name
+- Inode
+
+
 ## Symlinks
 **Hard** vs **Soft**. Hard is a literal copy of the file but keep the same inode info, while soft is just a pointer.
 
@@ -106,6 +122,26 @@ Disk/partition can be used **raw** (no file system) or can be **formatted**. The
 > ![](Pasted%20image%2020250505144352.png)
 
 
+### Layout
+![](Pasted%20image%2020250505155546.png)
+
+
+- **Boot block**
+	- Contains initial bootstrap program to load the OS
+	- Typically the first sector reads another program from the next few sectors
+- **Super block** - state of the file system
+	- Type -> ext3,ext4,FAT, etc.
+	- Size -> Number of blocks
+	- Block size
+	- Block group information -> number of block groups in file system
+	- Free block count
+	- Free inode count
+	- Inode size
+	- FS mount info
+	- Journal info
+
+### Free space management
+Unix uses a bitmap to show free disk blocks. Zero=free, one=in use
 ## Access lists and groups
 Read, write and execute.
 Three classes of users on Linux
@@ -114,6 +150,90 @@ Three classes of users on Linux
 3. Public  -> 1 (X)
  
 
+## Blocks
+The IDs of data blocks are stored in [INodes](File%20Systems%20Management.md#INodes), the IDs of the first 12 blocks are stored in direct reference fields.
+
+![](Pasted%20image%2020250505154746.png)
+
+
+
+### Allocation
+- Contiguous -> Stored in a single block
+- Linked Allocation -> blocks contain a pointer to the next one (slower access)
+- Indexed -> Each file has an index block that stores pointers to all its data blocks
+
+
+### Groups
+Subdivision of the entire disk or partition
+Has:
+- A block bitmap
+- An inode bitmap
+- An inode table holding the actual inodes
+
+> [!INFO]
+> Default block group size in ext4 is 128MB
+
+## Journaling
+Ensure the integrity of the file system by keeping track of changes before they are actually applied to the main file system
+
+Phases:
+- Write-ahead logging  -> before any changes are made to the file system
+- Commit -> shit actually happens
+- Crash recovery -> we can replay the journal to apply any uncommitted changes
+
+Types:
+- Write-Ahead Logging (WAL) -> logs changes before they are applied to the file system 
+- Metadata journaling -> only metadata is logged. Metadata is restored to a consistent state if crash.
+- Full journaling -> both
+
+
+## Example: EXT4
+- Journaling
+- Larger file and volume sizes
+- Extents -> range of contiguous blocks, reduces fragmentation
+- Multiblock allocator -> multiple blocks at once
+- `fsck`, optimized file system check
+- Pre-allocation
+- Checksums -> ensure integrity
+
+## Example: Windows FS
+### FAT(32)
+File allocation table.
+
+No hard links :C. Directory contains:
+- File name -> can be up to 8 characters and extension up to 3
+- Attributes (one byte)
+![](Pasted%20image%2020250505160518.png)
+
+- File size -> four byte field for filesize in bytes. Max. 4GB
+- ID of first block (4 byte)
+- File size
+
+Obviously this is trash since it cannot be used with disk of very large capacities. Windows introduced clustering 4,8,16 blocks together.
+
+The table itself is a list of blocks where many links are created and stored. Each entry is 4 bytes. List of empty blocks is also stored.
+
+![](Pasted%20image%2020250505161031.png)
+
+
+
+
+
+
+
+
+#### Free blocks list
+Stores a value for each cluster which can indicate:
+- `0x00000000` -> Free cluster
+- Next cluster number -> Cluster is allocated and points to the next one
+- `0xFFFFFFF8` - `0xFFFFFFFF` -> EOF
+- `0xFFFFFFF7` -> bad cluster
+
+To find a free block we just need to search for the first available cluster. We keep the last allocated cluster, optimizing search time.
+### NTFS
+New Technologies File System
+
+
 
 
 ---