Mem hierarchy
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The document discusses disk-based storage technologies and how they fit within the memory hierarchy. It describes the components of a disk drive, including platters, read/write heads, actuators, and electronics. Details are provided on disk geometry, with tracks divided into sectors, and the steps involved in disk access, including seek time, rotational latency, and data transfer time.
Mem hierarchy
- 1. 15-213 “ The course that gives CMU its Zip!” Disk-based Storage Oct. 23, 2008 Topics Storage technologies and trends Locality of reference Caching in the memory hierarchy lecture-17.ppt
- 2. Announcements Exam next Thursday style like exam #1: in class, open book/notes, no electronics 2 15-213, F’08
- 3. Disk-based storage in computers Memory/storage hierarchy Combining many technologies to balance costs/benefits Recall the memory hierarchy and virtual memory lectures 3 15-213, F’08
- 4. Memory/storage hierarchies Balancing performance with cost Small memories are fast but expensive Large memories are slow but cheap Exploit locality to get the best of both worlds locality = re-use/nearness of accesses allows most accesses to use small, fast memory Capacity Performance 4 15-213, F’08
- 5. An Example Memory Hierarchy Smaller, L0: faster, registers CPU registers hold words and retrieved from L1 cache. costlier L1: on-chip L1 (per byte) cache (SRAM) L1 cache holds cache lines retrieved from the L2 cache storage memory. devices L2: off-chip L2 cache (SRAM) L2 cache holds cache lines retrieved from main memory. L3: main memory Larger, (DRAM) Main memory holds disk slower, blocks retrieved from local and disks. cheaper local secondary storage L4: (per byte) (local disks) Local disks hold files storage retrieved from disks on devices remote network servers. L5: remote secondary storage (tapes, distributed file systems, Web servers) 5 From lecture-9.ppt 15-213, F’08
- 6. Page Faults A page fault is caused by a reference to a VM word that is not in physical (main) memory Example: An instruction references a word contained in VP 3, a miss that triggers a page fault exception Physical memory Virtual address Physical page (DRAM) number or VP 1 PP 0 Valid disk address VP 2 PTE 0 0 null VP 7 1 VP 4 PP 3 1 0 1 0 null Virtual memory 0 (disk) PTE 7 1 VP 1 Memory resident VP 2 page table VP 3 (DRAM) VP 4 VP 6 6 From lecture-14.ppt VP 7 15-213, F’08
- 7. Disk-based storage in computers Memory/storage hierarchy Combining many technologies to balance costs/benefits Recall the memory hierarchy and virtual memory lectures Persistence Storing data for lengthy periods of time DRAM/SRAM is “volatile”: contents lost if power lost Disks are “non-volatile”: contents survive power outages To be useful, it must also be possible to find it again later this brings in many interesting data organization, consistency, and management issues take 18-746/15-746 Storage Systems 7 we’ll talk a bit about file systems next 15-213, F’08
- 8. What’s Inside A Disk Drive? Spindle Arm Platters Actuator Electronics SCSI connector Image courtesy of Seagate Technology 8 15-213, F’08
- 9. Disk Electronics Just like a small computer – processor, memory, network iface • Connect to disk • Control processor • Cache memory • Control ASIC • Connect to motor 9 15-213, F’08
- 10. Disk “Geometry” Disks contain platters, each with two surfaces Each surface organized in concentric rings called tracks Each track consists of sectors separated by gaps tracks surface track k gaps spindle sectors 10 15-213, F’08
- 11. Disk Geometry (Muliple-Platter View) Aligned tracks form a cylinder cylinder k surface 0 platter 0 surface 1 surface 2 platter 1 surface 3 surface 4 platter 2 surface 5 spindle 11 15-213, F’08
- 12. Disk Structure Read/Write Head Arm Upper Surface Platter Lower Surface Cylinder Track Sector Actuator 12 15-213, F’08
- 13. Disk Operation (Single-Platter View) The disk surface The read/write head spins at a is attached to the end fixed of the arm and flies over rotational rate the disk surface on a thin cushion of air spindle spindle spindle spindle By moving radially, the arm can position the read/write head over any track 13 15-213, F’08
- 14. Disk Operation (Multi-Platter View) read/write heads move in unison from cylinder to cylinder arm spindle 14 15-213, F’08
- 15. Disk Structure - top view of single platter Surface organized into tracks Tracks divided into sectors 15 15-213, F’08
- 16. Disk Access Head in position above a track 16 15-213, F’08
- 17. Disk Access Rotation is counter-clockwise 17 15-213, F’08
- 18. Disk Access – Read About to read blue sector 18 15-213, F’08
- 19. Disk Access – Read After BLUE read After reading blue sector 19 15-213, F’08
- 20. Disk Access – Read After BLUE read Red request scheduled next 20 15-213, F’08
- 21. Disk Access – Seek After BLUE read Seek for RED Seek to red’s track 21 15-213, F’08
- 22. Disk Access – Rotational Latency After BLUE read Seek for RED Rotational latency Wait for red sector to rotate around 22 15-213, F’08
- 23. Disk Access – Read After BLUE read Seek for RED Rotational latency After RED read Complete read of red 23 15-213, F’08
- 24. Disk Access – Service Time Components After BLUE read Seek for RED Rotational latency After RED read Seek Rotational Latency Data Transfer 24 15-213, F’08
- 25. Disk Access Time Average time to access a specific sector approximated by: Taccess = Tavg seek + Tavg rotation + Tavg transfer Seek time (Tavg seek) Time to position heads over cylinder containing target sector Typical Tavg seek = 3-5 ms Rotational latency (Tavg rotation) Time waiting for first bit of target sector to pass under r/w head Tavg rotation = 1/2 x 1/RPMs x 60 sec/1 min e.g., 3ms for 10,000 RPM disk Transfer time (Tavg transfer) Time to read the bits in the target sector 25 15-213, F’08 Tavg transfer = 1/RPM x 1/(avg # sectors/track) x 60
- 26. Disk Access Time Example Given: Rotational rate = 7,200 RPM Average seek time = 5 ms Avg # sectors/track = 1000 Derived average time to access random sector: Tavg rotation = 1/2 x (60 secs/7200 RPM) x 1000 ms/sec = 4 ms Tavg transfer = 60/7200 RPM x 1/400 secs/track x 1000 ms/sec = 0.008 ms Taccess = 5 ms + 4 ms + 0.008 ms = 9.008 ms Time to second sector: 0.008 ms Important points: Access time dominated by seek time and rotational latency First bit in a sector is the most expensive, the rest are free SRAM access time is about 4 ns/doubleword, DRAM about 60 ns 26 15-213, F’08 ~100,000 times longer to access a word on disk than in DRAM
- 27. Disk storage as array of blocks …5 6 7 12 23 … OS’s view of storage device (as exposed by SCSI or IDE/ATA protocols) Common “logical block” size: 512 bytes Number of blocks: device capacity / block size Common OS-to-storage requests defined by few fields R/W, block #, # of blocks, memory source/dest 27 15-213, F’08
- 28. Page Faults A page fault is caused by a reference to a VM word that is not in physical (main) memory Example: An instruction references block” number can be 3, a “logical a word contained in VP miss that triggers a page fault exception in page table to remembered Virtual address identify disk location(DRAM) Physical page for pages Physical memory number or not resident in main memory VP 1 PP 0 Valid disk address VP 2 PTE 0 0 null VP 7 1 VP 4 PP 3 1 0 1 0 null Virtual memory 0 (disk) PTE 7 1 VP 1 Memory resident VP 2 page table VP 3 (DRAM) VP 4 VP 6 28 From lecture-14.ppt VP 7 15-213, F’08
- 29. In device, “blocks” mapped to physical store Disk Sector (usually same size as block) 29 15-213, F’08
- 30. Physical sectors of a single- surface disk 30 15-213, F’08
- 31. LBN-to-physical for a single- surface disk 4 5 3 16 17 6 15 28 29 18 27 30 40 41 2 39 42 7 14 19 26 38 43 31 37 44 32 25 1 13 36 45 20 8 4 46 24 7 33 12 35 34 21 0 22 9 23 11 10 31 15-213, F’08
- 32. Disk Capacity Capacity: maximum number of bits that can be stored Vendors express capacity in units of gigabytes (GB), where 1 GB = 10 9 Bytes (Lawsuit pending! Claims deceptive advertising) Capacity is determined by these technology factors: Recording density (bits/in): number of bits that can be squeezed into a 1 inch linear segment of a track Track density (tracks/in): number of tracks that can be squeezed into a 1 inch radial segment Areal density (bits/in2 ): product of recording and track density 32 15-213, F’08
- 33. Computing Disk Capacity Capacity = (# bytes/sector) x (avg. # sectors/track) x (# tracks/surface) x (# surfaces/platter) x (# platters/disk) Example: 512 bytes/sector 1000 sectors/track (on average) 20,000 tracks/surface 2 surfaces/platter 5 platters/disk Capacity = 512 x 1000 x 80000 x 2 x 5 = 409,600,000,000 = 409.6 GB 33 15-213, F’08
- 34. Looking back at the hardware CPU chip register file ALU main bus interface memory 34 15-213, F’08
- 35. Connecting I/O devices: the I/O Bus CPU chip register file ALU system bus memory bus I/O main bus interface bridge memory I/O bus Expansion slots for other devices such USB graphics disk as network adapters controller adapter controller mousekeyboard monitor disk 35 15-213, F’08
- 36. Reading from disk (1) CPU chip CPU initiates a disk read by writing a READ register file command, logical block number, number of blocks, and destination memory address to a ALU port (address) associated with disk controller main bus interface memory I/O bus USB graphics disk controller adapter controller mousekeyboard monitor disk 36 15-213, F’08
- 37. Reading from disk (2) CPU chip Disk controller reads the sectors and register file performs a direct memory access (DMA) transfer into main memory ALU main bus interface memory I/O bus USB graphics disk controller adapter controller mousekeyboard monitor disk 37 15-213, F’08
- 38. Reading from disk (3) CPU chip When the DMA transfer completes, the register file disk controller notifies the CPU with an interrupt (i.e., asserts a special “interrupt” ALU pin on the CPU) main bus interface memory I/O bus USB graphics disk controller adapter controller mousekeyboard monitor disk 38 15-213, F’08
Editor's Notes
- #4: *
- #8: *
- #10: No copyright needed – scanned by Erik Riedel.
- #16: Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: None. Kill text and arrows
- #17: Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: None. Kill text and arrows
- #18: Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: None. Kill text and arrows
- #19: Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: None. Kill text and arrows
- #20: Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: None. Kill text and arrows
- #21: Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: None. Kill text and arrows
- #22: Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: None. Kill text and arrows
- #23: Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: None. Kill text and arrows
- #24: Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: None. Kill text and arrows
- #25: Goal: Show the inefficeincy of current disk requests. Conveyed Ideas: Rotational latency is wasted time that can be used to service tasks Background Information: None. Slide Background: None. Kill text and arrows