What is new with rama
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What is new with RAMA?. Naomi Avigdor. Review. RAMA is a network file system Disks are divided into fixed size lines (rings) File blocks are hashed to a specific disk and a specific line on that disk No need to search inodes for file location

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What is new with RAMA?

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What is new with rama

What is new with RAMA?

Naomi Avigdor

What is new with RAMA?


Review

Review

  • RAMA is a network file system

  • Disks are divided into fixed size lines (rings)

  • File blocks are hashed to a specific disk and a specific line on that disk

  • No need to search inodes for file location

  • Files have parity protection for fault tolerance (optional)

What is new with RAMA?


Overflow

Overflow

  • When a line is full, the entire disk is full, regardless of data in other lines

  • What to do? Reserve overflow area on each disk

  • Simulation:

  • How full is the disk before overflow happens?

  • How much overflow area is needed?

  • Smaller / Larger systems? Any effect on overflow?

What is new with RAMA?


Overflow math

Overflow Math

C = capacity of each line

N = number of disks * number of lines per disk

T = total blks at ov point

fullness = total blks at ov point / total space

= T / N*C

What is new with RAMA?


Explanation of results

Explanation of Results

What is new with RAMA?


Conclusions

Conclusions

  • Hash function is very good

  • 1 overflow line is sufficient

What is new with RAMA?


What is parity

What is parity?

(single) Fault tolerance by creating a sum (mod2) of the data and saving it on another disk.

Ex: d1 =3, d2=5, d3 = 2, d4 = 1

p = 3 xor 5 xor 2 xor 1 = 5

D1, d2, d3, d4, p, each reside on a different disk (why?)

D1, d2, d3, d4, p, together are called a stripe

If d3 is lost: restore d3 by

D3 = p xor d1 xor d2 xor d4 = 5 xor 3 xor 5 xor 1 = 2

If d3 is changed: new p = old p xor old d3 xor new d3

or: new p = old d1 xor old d2 xor new d3 xor

old d4

What is new with RAMA?


What happens on a write

What happens on a write?

If d3 is changed:

Traditional:

new p = old p xor old d3 xor new d3

Cost = read old p, read old d3, write new p, write new d3 = 2 reads + 2 writes

RAMA:

New p = read old d1, d2, d4, write new p, write new d3 = 3 reads + 2 writes

What is new with RAMA?


What happens on a write cont

What happens on a write (Cont.)?

If d3, d4 are changed:

Traditional:

new p = repeat as before, two times

Cost = 4 reads + 4 writes

RAMA:

New p = read old d1, d2, write new p, write new d3, write new d4

Cost = 2 reads, 3 write

What is new with RAMA?


What happens on a write cont1

What happens on a write (Cont.)?

If d1, d3, d4 are changed:

Traditional:

new p = repeat as before, three times

Cost = 6 reads + 6 writes

RAMA:

New p = read old d2, write new p, write new d1, d3, d4

Cost = 1 reads, 4 write

What is new with RAMA?


Conclusion rama parity

Conclusion – RAMA parity

  • Parity updates can be delayed without the data writes being delayed for more efficiency (since old data is not needed)

  • For applications where minimum changes are made to each stripe, RAMA may be slower.

What is new with RAMA?


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