cs162 March 12, 2007


Newer tapes (IBM 2480 types) are cartidges
hold about same capacity as 2400' tape. 18 track.
220MBytes. Cost (end user, 1990, IBM -$95000)
	Example 1992 Cranel:
		18 tracks. 3480 compatible 540 foot tape .5 inch
		wide cartidge is 1" x 4.3" x 5" MTBF 15000 hrs.
		tape spead - 1m/s transfer 3MB/sec 18 tracks
		cartridge capacity 20MB
	used in automated tape libraries.
	
DECtape- "randomly" readable and writeable. Used on DEC minicomputers, as cheap addressable storage. Now obsolete 60-70.

DAT tape - 4mm tape in DAT cartridge, (smaller than audio casette) capacity about 1-2Gigabytes
 
Uses 3 level error correction --errors are 1 in 10**15

Uses embedded subcodes to find files and tracks. Tape is blocked in 512 KB blocks. can fast forward to approx block. data also organized into groups of 126632 bytes. each group contains 22 logical data frames of fixed capicity

There is a mode which permits random read/write

head/tape spead 123 inch/sec
track angle 6 degrees
uses 2 read heads and 2 write heads

Example 1993 hp
	2GB 183KB/sec, 30sec seek,  3.8 wats, mtbf 50000hrs variable data compression

Exabyte Tape--8mm tape in cartridge HOlds 2.5-5.0 Gbytes
1995 5 gig, 5000 Kbytes/second, 9tracks, tape is 8mm, 7Mbytes/spuare inch tape is .5 inch/sec, 246Kb sustaind data rate

Example Cranel transer: 1.5Mbyte/sec 35million bits/sq in. .5 in per second of tape speed, rotor 18000, MTBF 20000 hrs error rate 1/10**13

DLT tape (Digital Linear Tape (quantum 2004))
	records data in serpentine pattern
	4.1x4.1x1
	length 2000'
	width .5"
>30 years
250000hrs MTBF
up to 300GB/tape before compression
up to 36 MB/sec burst transfer rate up to 200MB/sec

typical track density is around 1490 tpi
power(quantum) 32 watts
average file access time: 79sec(quantum)
low end version: capacity 40 GB 3MB/sec; 168 tracks, 123KBits/inch, 336 track/inch. MTBF 200000hrs 15 watts

Other tapes
	two general formats
		linear
		hectical

tape issues
	tapes deteriorate over time
	variety of formates
	slow
	high end drives are expensive
	cheapest way to save massive ammounts of data
	
(hard) disk
	technology similar to tape. head float(Fly) over disk surface, at a distance about 1 micron 
	circular path around disk is called a track
	record is block of data that has extra bits for error correction--self identifying
	
	switching from one head to another-->need to do another seek
	
	CompactFlash type I already established as leading storage form factor for cameras, PDAs
			type II intro by CFA 12/98 is identical to Type I except height increased from 3.3 mm to 5 mm
			
	microdrive brings high capacity
	
	
	if disk beomes contaminated, head can crash
	
	number of platters(2 surfaces per platter) cylinders, tracks, sectors, bytes per track, ect are all variable
	
	data can be written in variable size or fixed size blocks(sectors) IBM style high end disks permit variable size blocks . most other machines use fixed sectors
	
	records is what we as programmer know--sectors is what computer knows
	
	material written looks like tape block, inner-block gap, key field, control info, plus actual data. overhead for variable blocks is about 50-100 bytes plus IRG. smaller head for fixed blocks
	
	most disks used to be removable. now no hard disks are removeable (except XIP (100,200, 750MB) JAZZ 2.3 GB disk IOMEGA 
	
	CKD Block--count key data--old IBM--really expensive sell 10s of thousands out of 100 million disks sold per year
	
	
Floppy Disk 1.44 MB 60KB/sec

To do a disk I/O (internal)
	seek: give cylinder and track address--move read/write head to correct track
	
	set sector--wait for disk to rotate desired sector into posisiton (often channel can let go of disk while this happens. interrupt generated when right place is reached.) rational Latency
	
	read or write sector while it spins by
	
	modern disk controllers, all of this is built in, your data issues the I/O with a block number(linear between 0 and size of disk) and the seek/set sector/read-write are all done invisibly to the CPU
		The embedded cod(microcode) in the disk controller is huge. 100K-200K lines of assembler
		
RPS Miss
	problem with older disk is that path from disk head to CPU had to be available to do read/write.
		so if the path wasn't available when the sector rotated under the head, the I/O coul not start, and an RPS Miss occurred. (RPS-Rotational Position Sensing) needed to wait for another entire rotation
		
5-10% better seek time per year

Buffers increased relative I/O

noise goes up at least with square of speed

Microdrive
	3600 rpm, 260mA


49% increase in areal density per year

rpm up 9% year 

early 1990s internal data rate became much better

Cost for both disk drive and system level(whole package) has been steadily going down

Storage system volumetric density has been steadily increasing

2million sq feet in early 60s to 2 sq ft in 2000

data rate over time has been increasing from mechanics

head to media has been decreasing steaply 5yrs ago 10 nanometers. sensitivity of head, height of head, accuracy of arm, texture of medium

Recording density going up