1. Storage Systems
Sudhanva Gurumurthi

2. Abstraction Split up the design problem into several layers
At each intermediate layer:
Use what its lower layer provides to do something
Hide the characteristics of the lower layer to the one above.
Functionality
Physics 2

3. Associating Meaning to Physical Phenomena
Presence or absence of current defines operating states
No Current => “Off”
Current Flowing => “On”
Abstraction: Switch
We can implement this abstraction using a transistor 3

4. Abstracting Electricity via Transistors
A processor is:
A collection of transistors connected by wires, where each transistor is either in the on/off state 4

5. Requirements for Storage
Need a non-volatile medium for housing data
Retain data when external power is removed
Needs to keep the data for many years
Low Cost
$/GB

6. Data Representation for Storage
Electrical Representation:
No Current => “Off”
Current Flowing => “On”
This representation does not work for storage due to the non-volatility requirement

7. Magnetism! Use magnetic polarity to represent data Ferromagnetism
Retain magnetization even after the external magnetic field is removed.
Ferromagenetism – Materials whose magnetic dipole moments remain aligned even after the external magnetic field is removed.
Ferroelectric material – Dielectric materials that can be given a permanent electrical polarization even after the external electric field is removed.
Image Source:

8. Hysteresis Loop Logic “1” Logic “0”
Ferromagenetism – Materials whose magnetic dipole moments remain aligned even after the external magnetic field is removed.
Ferroelectric material – Dielectric materials that can be given a permanent electrical polarization even after the external electric field is removed.
Logic “0”
Image Source:

9. Hard Disk Drive (HDD)
Faraday’s Law
Magnetic Induction

10. A Magnetic ‘Bit’ Logic “0” Logic “1”
Region of grains of uniform magnetic polarity
Logic “1”
Boundary between regions of opposite magnetization
Source:

11. Abstracting the Magnetics
Bits are grouped into 512-byte sectors
To read data on a different track, we need to move the arm (seek)
Source:

12. Disk Seeks
Arm
Platter

13. Disk Seek “Flight Plan”
Speedup
Arm accelerates
Coast
Arm moves at maximum velocity (long seeks)
Slowdown
Arm brought to rest near desired track
Settle
Head is adjusted to reach the access the desired location

14. Using the Disk Drive
Hardware/Software Interface of the disk drive (Architecture)
Instructions/Commands to read and write data from/to various sectors
Memory and Registers to buffer the data between the electronics and the platters
Other electronic components:
Error Correcting Code
Motor drivers

15. Memory Mapped I/O
Can read and write to disk just like normal memory through address ranges.
The addresses to these devices may not need to go through address translation
OS is the one accessing them and protection does not need to be enforced
There is no swapping/paging for these addresses.

16. Reading Data from Disk 0x00…0 Memory Bus Main Memory 0x0ff..f 0x100…0
I/O
Bus
RAM
Controller
0x1ff….f

17. Reading a Sector from Disk
Processing On the CPU
Store [Command_Reg], READ_COMMAND
Store [Track_Reg], Track #
Store [Sector_Reg], Sector #
/* Device starts operation */
L: Load R, [Status_Reg]
cmp R, 0
jeq
/* Data now available in disk RAM */
For i = 1 to sectorsize
Memtarget[i] = MemOnDisk[i]
CPU Overhead!
Instead, block/switch to
other process and let an
interrupt wake you up.
Again too much CPU
overhead!

18. Direct-Memory Access (DMA)
Store [Command_Reg], READ_COMMAND
Store [Track_Reg], Track #
Store [Sector_Reg], Sector #
Store [Memory_Address_Reg], Address
/* Device starts operation */
P(disk_request); …
/* Operation complete and data is
now in required memory locations*/
Assume that the DMA controller is integrated into the disk drive
Called when DMA raises
interrupt after
Completion of transfer
ISR() {
V(disk_request); }

19. Memory Technologies
Volatile
Flash Memory
Non-Volatile

20. Solid State Disk (SSD) Disks that use Flash Memory No moving parts
Less power and heat
Quiet operation
Shock and vibration tolerant
Drop-in replacement for disks
Image Source: L. Waldock, “Intel X-25M Solid-State Drive”, Reghardware, September 2008.

21. The Architecture of an SSD
Source: Agrawal et al., “Design Tradeoffs for SSD Performance”, USENIX 2008

22. Characteristics of Flash Memory
Reads/Writes done at the page granularity
Page Size: 2-4 Kilobytes
Writes can be done only to pages in the erased state
In-place writes are very inefficient
Erases done at a larger block granularity
Block Size: pages
Time for Page-Read < Page-Program < Block-Erase
Limited endurance due to programs and erases
These issues are handled by the Flash Translation Layer (FTL) inside the SSD

23. Logical Block Map
Each write to a logical disk Logical Block Address (LBA) happens to a different physical Flash page
Need a LBA -> Flash page mapping table
Mapping table stored in SSD DRAM and reconstructed when booting
Target flash page for a LBA write is chosen from an allocation pool of free blocks

24. Cleaning
Writes leave behind blocks with stale copies of the page (superseded pages)
Invoke garbage collection to erase these blocks and add them to the allocation pool
page-size < block-size => copy non-superseded pages in the block to another block before erasure

25. Cleaning and Wear-Leveling
Choose blocks that have the highest number of superseded pages to reduce the number of copies
SSD capacity is overprovisioned to mask garbage collection overheads
Wear-Leveling
Distribute the program/erase cycles evenly over all the blocks in the SSD