J65nko just reminded me (from
this post in another thread) that for 4096-byte sector disk drives that map to 512-byte sector devices (called "
Advanced Format 512e" devices) there is additional latency on write operations that write 512-byte sectors, for a process called Read-Modify-Write ("RMW"):
- Seek to the cylinder
- Wait for the sector to rotate under the head
- Read the 4096-byte sector into cache
- Modify the 512-byte sub-sector in cache
- Wait for the sector to rotate under the head
- Write the 4096-byte sector.
The alignment issue J65nko refers to in his post is to when a sequential 512-byte write from the OS crosses the 4096-byte physical sector boundary, requiring a second RMW operation, and all the extended milliseconds that requires for RPS latency.