TweetFollow Us on Twitter

File, Disk Servers
Volume Number:3
Issue Number:4
Column Tag:Networking Issues

File Servers versus Disk Servers

By Tim Maroney, Software Designer, Centram Systems West

Your kindly editor, Dave Smith, has invited us to clear up some common misperceptions about TOPS, and generally dispel the fog of confusion that surrounds the whole area of networked file systems. A widely distributed Mac magazine recently ran a short piece on the difference between file servers and disk servers that was almost completely wrong, and at trade shows one often hears sales people giving out incorrect information. This article should help people to navigate through the dimly-lit coral reefs of networking.

There are three main approaches to sharing files over a network: file transfer, disk service, and file service. (There is also disk transfer, known to initiates as "the Frisbee method".)

The most venerable approach is file transfer. Most programmers have used file transfer over phone lines; it's much the same over a network. Instead of dialing a phone number, one types in or selects a machine name, but the basic sequence of operations is the same. The user asks to send or receive a file to or from a remote system. Cooperating software on both machines breaks the file up into small packets of data and reliably transfers and acknowledges the packets over the serial line or network. Each packet is sent with a "checksum" or "cyclic redundancy check" value derived by performing a sequence of arithmetic operations on the bytes in the packet. If the receiving machine finds that a packet doesn't match its checksum or CRC, it asks for the packet to be sent again. In this way, the entire file is sent with guaranteed correctness. On serial lines, the protocol is likely to be Kermit or XMODEM; on a network, it is likely to be FTP (File Transfer Protocol).

An open secret of networking is that there is no such thing as a perfect guarantee of correctness. It is possible for a packet to be completely garbaged by line noise, but to coincidentally fall together into an acceptable packet with a valid checksum. It is also possible, as Gamow pointed out, for all the molecules of air in a room to randomly wind up in the same corner of the room and leave any people in the room gasping in a vacuum. It isn't particularly likely, and neither is a network or line error that yields a valid checksum or CRC value.

File transfer is adequate for many applications, particularly keeping libraries of software or literature which people want to download to their machines. However, there are a number of file sharing applications which require a more dynamic approach. For instance, if you have a distributed database, you don't want to have to download it to your machine, make changes, and then upload it back to the original machine. In a multiple-engineer programming product, it might be desirable to keep the sources on a central machine and have everyone work from the same copies, while actually using their own microcomputers. Sometimes you have only one hard disk but three people need to have a few megabytes each at the same time. And so forth. For these kinds of applications, disk service or file service is more suitable.

Disk service and file service look similar to a human user, but the implementations are different in significant ways. In both cases, though, the idea is to make disk storage that is connected to another machine seem to be directly connected to your machine. On the Mac, that means (from a user's perspective) that a remote disk volume appears with a disk icon in the Finder, and can also be seen inside the Standard File Package, so that the files on the remote disk can be used just like local files. The term "transparency" is usually used to refer to this kind of file access; the fact that the file actually resides on another system is transparent (invisible) to software. In other words, transparency of disk or file service means that old programs still work, without having to put out new versions.

You can see that file transfer is actually a functional subset of disk service or file service. Network file transfer can be done in the Finder on the Mac using TOPS or MacServe, without requiring a special transfer utility; all you have to do is drag the remote file to a local (or even another remote) volume or folder.

In just about every operating system on the planet, there are two levels of file access. The programmer uses high-level file operations, like open, read, write, close, and so forth. High-level file operations are translated by the operating system into low-level disk operations involving physical disk block reads and writes. Low-level operations are usually structured as calls to any of several lookalike disk drivers, pieces of software in the OS that deal with the details of communicating with the disk controller. An operating system is associated with a particular disk format, which is the same from disk to disk. That is, regardless of whether you have a DataFrame or an HD20 connected to your Mac Plus, the first two blocks on the disk contain system startup information, the third volume information, and so forth, even though two different disk drivers are used to talk to the disks, and the disks represent their blocks differently at the physical level.

Disk service intercepts file operations at the level of the disk driver. File service, however, intercepts file operations in the high level operations. This leads to some important differences in the power and performance of the two approaches.

In disk service, a disk (or possibly a simulated disk) on a remote machine is accessed by the operating system just like a local disk; physical disk block reads and writes go directly over the network. Disk service uses a disk driver that goes to the network instead of to a local file device. You could say that the network is being used like a long SCSI cable. Disk service is very simple to implement; a friend once claimed that he could write a complete disk server in under two hours.

Clearly, disk service is bound by disk formats, and so it does not work very well, if at all, between different operating systems. A Mac and a PC want to see very different things on their disks. It is possible, though difficult and expensive, to let each understand the other's format; for instance, an external file system could be written on the Mac to understand PC-format disks. However, there is a combinatorial explosion associated with adding more formats. Each new system's format has to be implemented on each already supported system, requiring lots of coding effort, and lots of code space overhead on each machine.

Disk service does not easily permit file sharing between users. Disk service uses an existing, unmodified or only very slightly modified, disk format, the format that came with the operating system. Disk formats do not typically allow easy synchronization of multiple users, because they are intended to be used only by a single local machine. This means that only one person can mount a network disk at one time, unless elaborate operating system interceptions and synchronization protocols are developed. If such interceptions and protocols are done, then disk service is no longer simpler than file service; and this simplicity was really its only benefit.

One approach to inter-operating-system disk service is to partition a server's disk, and format each partition to the dictates of a different OS. To share files between operating systems, a special utility is used to copy across partitions. This is the approach used by 3-Com. This allows dynamic file sharing between machines using the same operating system (sometimes), but between operating systems it is really just file transfer. Using simulated disks has some of the same problems as partitioning; for instance, you could allocate a one megabyte file on a VMS system and use disk service software to make a Mac think this VMS file is really a block-structured Mac disk, but people on VMS are not going to be able to get any useful information out of the file without using special copying utilities.

In file service, high-level file system operations like open, read, write, lock, and so forth go over the network instead of disk block requests. In many file service protocols (e.g., TOPS, NFS, and CMU's VICE), a remote function call protocol is used for support: this allows one machine to make function calls that are executed on another machine. File service is usually very hard to implement well; TOPS took about two years, and VICE took even longer to become a usable system.

However, once file service is done, there are some very tangible benefits over disk service. The most tangible, to a naive end user, is that remote disks can be shared; more than one person can have access to the same directories and volumes at one time. No special synchronization protocol is needed.

Another very tangible benefit is an inter-operating-system capability. Most operating systems have similar high-level file operations, like open, read, seek, and so forth. There are differences, but they can almost always be bridged without losing compatibility. TOPS is a standard for file system operations regardless of operating system, and was simultaneously developed on two operating systems: that's how we were able to get the PC and the Mac to communicate, and why our UNIX and forthcoming OS implementations are proceeding smoothly. Some other file service protocols are less OS independent; for instance, VICE is very specific to 4.2bsd UNIX, and a new protocol, SNAP, had to be added to allow VICE machines to share files with microcomputers.

Another benefit is that considerably more clever and powerful things can be done in file service. VICE uses a whole-file local caching scheme that speeds up file access tremedously for workstations that have their own disks. A file service protocol can be extended easily to cope with the demands of new operating systems, without encountering the combinatorial explosion of disk service. Disk servers, because of the lack of file sharing, do not usually allow a machine to serve as both client and server, or to function as one node in a homogeneous network namespace; these things can be added to file service relatively easily.

File servers are often faster than disk servers. TOPS, a file server, is faster than MacServe, a disk server, according to InfoWorld (11/86) and MacWorld (10/86). This might seem puzzling, since disk service is simpler than file service. We aren't entirely sure, but we think that it is the result of disk service's need to pass directory and map blocks over the network to do directory, seek, and grow operations. In file service, this is all done locally on the server machine, accomplishing in a single network operation what takes two or more with disk service. Of course, local operations are faster than network operations, just as eating everything at the table is faster than walking to the kitchen to fetch each mouthful. Believe me, I have tried this many times. So contrary to first impressions, a file server can often be expected to perform better than a disk server.

Some brief design notes might be helpful. TOPS is a name used for both a network protocol and the TOPS product which implements the protocol. The TOPS protocol is built on a lower-level protocol called RFP, for Remote Function Protocol. Using RFP, it is possible to make function calls that will be executed on another system. RFP itself is built on top of the Appletalk Transaction Protocol (ATP), and will soon be ported to run on the Internet Transmission Control Protocol (TCP) as well. RFP is an asymmetrical protocol; it has a client end, which makes remote calls and receives their values, and a server end, which receives remote calls, executes them locally, and returns the result to the client that initiated the call.

The TOPS protocol is a set of function definitions that are passed over the network using RFP. These include functions to open files, read and write buffers, lock files and byte ranges, get information on files and directories, and so forth. When some software on the Mac makes a file system call that has to do with a remote file, this system call is intercepted and translated into a TOPS call. RFP's client end is then used to make this TOPS call remotely on the system where the file is actually stored. The RFP server end on the machine containing the file executes the TOPS call locally, which means calling the local file system, and returns the result to the client. Because everything goes through TOPS, the two machines may have completely different operating systems. All that is needed is for the TOPS client software to translate local file system operations into TOPS operations, and for the TOPS server to translate TOPS operations into local file system operations.

The Apple Filing Protocol (AFP) and Sun's Network File System (NFS) use somewhat similar designs. With AFP, a protocol called ASP (Apple Session Protocol) is used for remote function calling. Actually, ASP does less than RFP, since it does not itself interpret the data in the packets, deferring this to a sort of implied remote function call layer in AFP itself. (Pay attention; this will be on the test.) ASP sits on top of ATP; like AFP, it was co-developed by Centram and Apple. NFS uses a protocol called RPC (Remote Procedure Call), which uses a sub-protocol known as XDR (External Data Reference) to define its data formats. RPC sits on top of the Internet User Datagram Protocol (UDP).

There were some comments on TOPS from "MacoWaco" in the January 1987 issue of MacTutor which were not quite accurate regarding use of Apple's "File Structure". I don't know what he means by "Apple's File structure". The most likely interpretation seems to be the Apple Filing Protocol, AFP for short. This is an Apple protocol for network file service. The design of AFP is similar to the design of the TOPS protocol. AFP is still being refined within Apple; when it is finalized later this year, TOPS will become fully compatible with it. MacServe, however, cannot be compatible with AFP. Its disk service approach is fundamentally incompatible with the file service approach employed by both TOPS and AFP. Another possible interpretation of "Apple's File structure" pertains to disk format. TOPS uses disks as they are, with no modifications needed, while MacServe requires reformatting and partitioning disks before they can be used.

Appletalk runs at about one quarter megabit per second, because this is the fastest speed the SCC will handle without special clocking. Ethernet runs at three or ten megabits per second, twelve or forty times as fast. It should be noted, though, that most network protocol implementations cannot drive a network at a full bandwidth of multiple megabits per second; an Ethernet tends to be idle a lot of the time, but it's still effectively many times faster than Appletalk, but then again, many times more expensive to implement. The new Macs will allow customers to match their pocketbooks with their bandwidth requirements.


Community Search:
MacTech Search:

Software Updates via MacUpdate

Warhammer 40,000: Freeblade guide - How...
Warhammer 40,000: Freebladejust launched in the App Store and it lets you live your childhood dream of blowing up and slashing a bunch of enemies as a massive, hulking Space Marine. It's not easy being a Space Marine though - and particularly if... | Read more »
Gopogo guide - How to bounce like the be...
Nitrome just launched a new game and, as to be expected, it's a lot of addictive fun. It's called Gopogo, and it challenges you to hoparound a bunch of platforms, avoiding enemies and picking up shiny stuff. It's not easy though - just like the... | Read more »
Sago Mini Superhero (Education)
Sago Mini Superhero 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: KAPOW! Jack the rabbit bursts into the sky as the Sago Mini Superhero! Fly with Jack as he lifts impossible weights,... | Read more »
Star Wars: Galaxy of Heroes guide - How...
Star Wars: Galaxy of Heroes is all about collecting heroes, powering them up, and using them together to defeat your foes. It's pretty straightforward stuff for the most part, but increasing your characters' stats can be a bit confusing because it... | Read more »
The best cooking apps (just in time for...
It’s that time of year again, where you’ll be gathering around the dinner table with your family and a huge feast in front of you. [Read more] | Read more »
Square Rave guide - How to grab those te...
Square Rave is an awesome little music-oriented puzzle game that smacks of games like Lumines, but with its own unique sense of gameplay. To help wrap your head around the game, keep the following tips and tricks in mind. [Read more] | Read more »
Snowboard Party 2 (Games)
Snowboard Party 2 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Crowned the best snowboarding game available on the market, Snowboard Party is back to fulfill all your adrenaline needs in... | Read more »
One Button Travel (Games)
One Button Travel 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: “To cut a long story short, If you like interactive fiction, just go buy this one.” - “Oozes the polish that... | Read more »
Light Apprentice Volume 1 (Games)
Light Apprentice Volume 1 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Light Apprentice Volume 1 includes Chapters 1 to 4, all gathered in a new exclusive game. When life in the world of... | Read more »
The best games like Animal Crossing on m...
Animal Crossing amiibo Festival is out right now for the Wii U, reminding us of just how much fun that world can be. Or at least to go back and check in on our villages once in a while. [Read more] | Read more »

Price Scanner via

iMobie Releases its Ace iOS Cleaner PhoneClea...
iMobie Inc. has announced the new update of PhoneClean 4, its iOS cleaner designed to reclaim wasted space on iPhone/iPad for use and keep the device fast. Alongside, iMobie hosts a 3-day giveaway of... Read more
U.S. Cellular Offering iPad Pro
U.S. Cellular today announced that it is offering the new iPad Pro with Wi-Fi + Cellular, featuring a 12.9-inch Retina display with 5.6 million pixels — the most ever in an iOS device. U.S. Cellular... Read more
Newegg Canada Unveils Black Friday Deals for...
Newegg Canada is offering more than 1,000 deep discounts to Canadian customers this Black Friday, available now through Cyber Monday, with new deals posted throughout the week. “Black Friday is... Read more
Black Friday: Macs on sale for up to $500 off...
BLACK FRIDAY B&H Photo has all new Macs on sale for up to $500 off MSRP as part of their early Black Friday sale including free shipping plus NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $... Read more
Black Friday: Up to $125 off iPad Air 2s at B...
BLACK FRIDAY Walmart has the 16GB iPad Air 2 WiFi on sale for $100 off MSRP on their online store. Choose free shipping or free local store pickup (if available): - 16GB iPad Air 2 WiFi: $399, save $... Read more
Black Friday: iPad mini 4s on sale for $100 o...
BLACK FRIDAY Best Buy has iPad mini 4s on sale for $100 off MSRP on their online store for Black Friday. Choose free shipping or free local store pickup (if available): - 16GB iPad mini 4 WiFi: $299.... Read more
Black Friday: Apple Watch for up to $100 off...
BLACK FRIDAY Apple resellers are offering discounts and bundles with the purchase of an Apple Watch this Black Friday. Below is a roundup of the deals being offered by authorized Watch resellers:... Read more
Black Friday: Target offers 6th Generation iP...
BLACK FRIDAY Save $40 to $60 on a 6th generation iPod touch at Target with free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary: -... Read more
Black Friday: Walmart and Target offer iPod n...
BLACK FRIDAY Walmart has the 16GB iPod nano (various colors) on sale for $119.20 on their online store for a limited time. That’s $30 off MSRP. Choose free shipping or free local store pickup (if... Read more
Black Friday deals on the Apple Watch and App...
Apple resellers are offering discounts and bundles with the purchase of an Apple Watch this Black Friday weekend. Below is a roundup of the deals being offered by authorized Watch resellers: Apple... Read more

Jobs Board

Storefront Operations Coordinator, *Apple* -...
# Storefront Operations Coordinator, Apple -Latin America Job Number: 43587750 Miami, Florida, United States Posted: Oct. 16, 2015 Weekly Hours: 40.00 **Job Summary** The Read more
*Apple* Enterprise / Government Professional...
# Apple Enterprise / Gove ment Professional Services Engineer Job Number: 42292976 Reston, Virginia, United States Posted: Aug. 18, 2015 Weekly Hours: 40.00 **Job Read more
iOS Wallet & *Apple* Pay Engineer - App...
# iOS Wallet & Apple Pay Engineer Job Number: 40586801 Santa Clara Valley, Califo ia, United States Posted: Nov. 16, 2015 Weekly Hours: 40.00 **Job Summary** The iOS Read more
Software Engineer, *Apple* Watch - Clock Fa...
# Software Engineer, Apple Watch - Clock Face Team Job Number: 44368761 Santa Clara Valley, Califo ia, United States Posted: Nov. 14, 2015 Weekly Hours: 40.00 **Job Read more
Administrative Assistant, *Apple* Online St...
# Administrative Assistant, Apple Online Store Job Number: 43992352 Santa Clara Valley, Califo ia, United States Posted: Nov. 9, 2015 Weekly Hours: 40.00 **Job Summary** Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.