CS E6998-10.  Advanced Topics in Network Storage Systems
			       March 30, 2004
			           Quiz #2


      Name:					         ID#:




Give short answers to the following questions:


(10 pts)
1. How does iSCSI achieve Direct Data Placement (DDP)?  Which of the
   network file systems we studied employs a similar technique?


iSCSI achieves DDP by incorporating a tag in the iSCSI PDU header.
Using this tag, an iSCSI receiver can match an incoming iSCSI PDU 
with the destination buffer and perform DDP.  The network file 
system that employs a similar technique (RDMA) is DAFS.

(See K. Meth, J. Satran, "The Design of the iSCSI Protocol", Section
2.5 for the full explanation).


(10 pts)
2. Which characteristic of FibreChannel makes it suitable for a
   Storage-Area Network (SAN)?


Short answer: Low-level support for reliable data transfer.

From C. Jurgens, "Fibre Channel: A connection to the Future", page
89, referring to the FC-2 layer of Fibre Channel: "The sequence is
also the recovery boundary of Fibre Channel.  When an error is
detected, Fibre Channel identifies the errant sequence and lets
that sequence (and subsequent sequences) be retransmitted.  This
unique capability enables Fibre Channel to provide reliable data
transfer without relying on an upper layer protocol like TCP/IP.
[This characteristic of Fibre Channel] makes [it] very desirable
for storage I/O"


(10 pts)
3. Which programming techniques can be used by applications accessing 
   remote storage to mask network delays typical in Internet-scale 
   wide-area networks?
   

Caching --> avoids network I/O

Application prefetching --> pipelines the network link

Asynchronous writes (write-back caching) --> masks the network delay

(For more details, see W.-T. Ng and others, "Obtaining High
Performance for Storage Outsourcing")


(10 pts)
4.  What are the primary benefits of Venti?


a. With the use of a hash of a block contents as the block
   identifier for read and write operations, Venti enforces
   a write-once policy, preventing accidental or malicious
   destruction of data.

b. Duplicate copies of a block can be coalesced, reducing
   storage consumption.

c. Since Venti uses magnetic disks, it has much better
   access time than other archival storage systems (e.g., 
   those based on tapes)