vault backup: 2025-05-05 20:41:16

Operating Systems/Mass Storage.md

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+## Hard disks (HDDs)
+Spin platters of magnetically-coated material under moving RW heads
+- Rotate at 60 to 250 times/s
+- Transfer rate is rate at which data flows between drive and pc
+- Positioning time (Random access time) to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency)
+
+![](Pasted%20image%2020250505201859.png)
+
+
+### Controller
+Receives data/ command from the OS, controls I/O operation and sends back results to OS
+![](Pasted%20image%2020250505201950.png)
+
+
+### I/O steps
+- Move head to track -> Seek Time (S)
+- Find sector -> Rotational Latency Time (r)
+- Transfer data to/from controller -> Block Transfer Time (btt)
+- Error checking and report to OS
+
+
+### Disk Formatting
+Each platter must receive a low-level format done by software, i.e. marking the cylinders and tracks, each containing a few sectors, with short gaps in between
+
+Each sector includes:
+- Preamble: starts with start of the sector bit pattern, the cylinder and sector numbers
+- Data: size of the data portion is determined by the formatting program -> 512 bytes usually
+- ECC: error correction, typically 16 bytes
+
+
+Numbering the sectors
+- Position of sector 0 on each track is offset from the previous track when the low-level format is laid down (cylinder skew)
+
+
+![](Pasted%20image%2020250505202532.png)
+
+![](Pasted%20image%2020250505203114.png)