# What is new with RAMA? - PowerPoint PPT Presentation

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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?

Naomi Avigdor

What is new with RAMA?

### 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

• 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

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

What is new with RAMA?

### Conclusions

• Hash function is very good

• 1 overflow line is sufficient

What is new with RAMA?

### 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?

If d3 is changed:

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.)?

If d3, d4 are changed:

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 (Cont.)?

If d1, d3, d4 are changed:

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

• 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?