.sh DISKS\(dg .FS \(dg Disk sizes shown throughout this document are in bytes of formatted space available. .FE The area of disks and disk controllers is one which has seen a great deal of change since the last revision of this paper in mid 1981. At that time we had no experience with Winchester technology disk drives. Now, after some painful experimentation, we have settled on a few Winchester products which fill our needs reliably. We no longer buy, or recommend, any removable media disk products. .LP The choice of available controllers is also wider and much improved. High quality controllers are available which interface to the native busses of 750s and 780s as well as the UNIBUS. In addition, DEC has introduced an entire new storage system architecture which places a great deal more function in the controller, incorporates a new controller-drive interconnect, and uses improved error correction algorithms. .LP First, we will discuss some of the major areas of change in disk/controller technology. We will then explore how these improve, or otherwise affect, our methods of doing business. Finally, we will consider some specific DEC and non-DEC products. .LP The availability of large capacity, low cost, high reliability Winchester technology disk drives has had an enormous impact on us. The rack mountable, 300 Megabyte or bigger disk which was always ``just around the corner'' is really here. It is hard to see how we got along without it. We can now put about 2 Gigabytes of storage in the same footprint that previously held 256 Megabytes. In addition, we consume and dissipate about 25% of the energy we did with older, removable media, drives. The prospective buyer should be warned, however, that not all ``winnies'' live up to expectations with respect to reliability. We are happy with the reliability of the equipment we describe here. If you want to try something else, be sure and have some long heart to heart talks with other users of the product. .LP Cost per Megabyte of disk storage is down significantly. Cost ranges from $30 to $110 per Megabyte for disks, not counting the price of the controller(s). This value depends on the size of the units purchased and the choice of vendor. Cost per unit storage in terms of both purchase price and cost to operate are a stronger inverse function of the total drive capacity than ever before. For example, the cost per Megabyte of the 456 Mbyte DEC RA81 is about 35% of that of the 121 Mbyte RA80. The reason for this becomes clear when the drives are examined; many of the components are identical. .LP The higher recording densities of new disk drives has also been a strong motivator in controller evolution. One technique for increasing the recording density of the drives has been to rely more heavily on sophisticated error correction and block remapping schemes. No large Winchester drive can be depended on to be ``error free.'' In fact, most the drives we use have uncorrectable media defects. These locations must be remapped using some combination of controller firmware and handler software. In addition, the higher bit rates of new disk drives demand faster serial logic in the controller interface. Many older disk controllers are limited to the burst transfer rate of 3330 style disks of about 1.25 Mbyte/sec. .LP Two types of controller have evolved for the newer, high bit density disks. The first is simply a version of the traditional SMD or Storage Module Drive interface reengineered for higher data rates. This type of interface characterizes all of the non-DEC controllers which have been produced for VAXes of the last few years. These controllers interface to the native busses of the VAX (SBI or CMI) where possible to allow the higher data rates available to be passed all the way through to memory. Where the controller must operate on a bus incapable of a continuous transfer rate as high as the disk, some amount of internal buffering is provided to maximize the amount of date transferred before the disk ``blows a rev''.* .FS * * By ``blowing a rev'', we mean a data transfer can not be completed without extraneous disk revolutions. This is mainly a function of the time required by a processor to service an interrupt, the bandwidth of the bus, and the buffering in the controller. With the 4.2BSD file system, disk controllers are now being extended to their limitations, and beyond. This has significantly influenced our concern for the their limitations as bandwidth suffers greatly when such an event takes place. .FE .LP Non-DEC controllers most often emulate the DEC RH11, RH750, or RH780 interface. Some support for error correction is provided by the controller although a substantial assist is usually required from the system driver. Remapping of uncorrectable media defects is entirely handled by the driver. All 4.2BSD device drivers support bad block remapping. In addition, error correction and remapping support is, optionally, available in the standalone utilities\(dg. .FS \(dg Due to limitations in the size of a binary image which may be placed on a boot cassette or floppy, the error correction and bad sector forwarding code is not included in the standalone utilities by default. .FE The only part of the system which does not gracefully handle errors or media defects is the first level bootstrap code used on 750s. .LP DEC has produced a very different type of controller, partially to deal with the challenges of higher density disk drives. This controller, the UDA50, is an example of DEC's long range plan for mass storage (this \*(lqplan\*(rq is called the Digital Storage Architecture, or DSA). One of the fundamental goals of DSA is to provide a standard set of disk \*(lqoperations\*(rq across a variety of storage products. With DSA it should be possible to construct standard drivers which know very little about the characteristics or geometry of the actual storage devices they are dealing with. In order to meet this goal, error correction, bad block forwarding, and even the mapping of logical blocks onto the physical disk are handled in the controller. Requests to the controller typically consist of logical block addresses and counts, along with a memory transfer address. Responses then contain either data or a failure message. The controller independently takes all possible measures to recover data before returning failure. .LP In addition to increasing the functionality of the controller, DSA specifies a new controller to drive interface. The Standard Disk Interface, or SDI, is capable of handling the transfer rates of any drive which DEC may produce in the foreseeable future. This interface is implemented using four electrically isolated radial mini-coax cables to each disk drive embedded in a tough rubber-like umbilical. .LP On 750 and 780 systems we are, or will be, buying either large (404 Mbyte) Fujitsu disk drives and Emulex SBI or CMI interfaced controllers, or DEC UDA50 controllers with (456 Megabyte) RA81 disk drives. The choice here is not clear as the two packages are both attractive and each has a different set of advantages. Although we do not currently have any UDA50/RA81s at Berkeley, several users of 4BSD do have them, and are very satisfied. In addition, we have visited Colorado Springs, where the drives are manufactured, and run benchmarks on them using an early version of 4.2BSD. The preliminary measurements support our optimism about the UDA50/RA81 combination, though we are not yet ready to publish these results (they will be available at a later time). .LP It is important not to place too much emphasis on raw performance issues when comparing products as similar in capabilities as the large disk choices presented here. Reliability, freedom from bugs, and ease of maintenance are equally if not more important to us. The value of the product in future configurations is also important. For example, the UDA50/RA81 disk system represents an early implementation of a new architecture. It incorporates many new features heretofore unavailable to us. In addition, it is expandable in the sense that the disk/controller interface is designed to handle future density increases which are not likely to be useable with the traditional SMD interface. On the otherhand, any implementation as new as the UDA50/RA81 is not as likely to be as bug free or as well understood as the traditional RH style interface architecture. .LP Table 1 indicates some of the tradeoffs as we now understand them. .KF .DS B .TS box; l l l ltiw(1.0i) ltw(2.0i) ltw(2.0i). Criterion UDA50/RA81 Emulex SC7?0/Fujitsu Eagle _ T{ .fi .na Initial Purchase Cost \- 750 T} T{ .fi .na UDA50 and 1st RA81 \- $57.00/Mbyte w/o additional UNIBUS adaptor; $70.00/Mbyte with UNIBUS adaptor T} T{ .fi .na SC750 and first Eagle \- $55.00/Mbyte T} .sp .5 T{ .fi .na Initial Purchase Cost \- 780 T} T{ .fi .na UDA50 and 1st RA81 \- $83.00/Mbyte with UNIBUS adaptor T} T{ .fi .na SC780 and 1st Eagle \- $65.00/Mbyte T} .sp .5 T{ .fi .na Cost for Incremental Addition T} T{ .fi .na Additional RA81s \- $41.00/Mbyte T} T{ .fi .na Additional Eagles \- $32.00/Mbyte T} .sp .5 T{ .fi Performance T} T{ .fi May be somewhat better in mixed request, multi drive environment due to ordering optimizations possible in controller; software handler at present is suboptimal T} T{ .fi Initial tests indicate 5-10% better single file throughput due to better sustained burst rate T} .sp .5 T{ .fi .na Maintenance Costs T} T{ .fi Very low \- $111/Mo. for 1st drive and controller (compare to $326 for RM05) T} T{ .fi .na Unknown but believed very low T} .sp .5 T{ .fi .na Mean Time Between Failure T} T{ .fi Too little experience available yet; RM80 is precursor of RA81 mechanically and has been quite good T} T{ .fi Not a lot of experience on these yet either; initial experience looks excellent (smaller Fujis are phenomenal; 30,000 MTBF!) T} .sp .5 T{ .fi .na Mean Time to Repair T} T{ .fi Designed for quick field removal of HDA; easy to repair T} T{ .fi Not as easy; more complex disassembly T} .sp .5 T{ .fi .na Sources of Maintenance T} T{ .fi DEC; maint. contract cheap, real, and available T} T{ .fi Not so clear; ask for exchange contract from vendor T} .sp .5 T{ .fi .na Robustness of Drive Interconnect T} T{ .fi Incredible \- electrical isolation and you could run over cables with a fork lift! Radial connection allows easy removal of a single drive T} T{ .fi Same old SMD flat cables; daisy chain T} .sp .5 T{ .fi .na Future Value T} T{ .fi Early implementation of new architecture; if it pans out, likely to be compatible with future, high performance, products; DEC resale high anyway T} T{ .fi High performance (stretched to limits) implementation of old interface standard; not likely to work again for next increase T} .sp .5 T{ .fi .na Cost to Integrate T} T{ .fi Handler is new; some initial bugs likely; probably a bug or two left in controller firmware too T} T{ .fi Well known interface; much more likely to be bug free T} .TE .sp 0.5 .ce Table 1. Large Disk System Comparison .DE .KE .LP When searching for less storage for smaller smaller systems, or where two arms are needed for performance and 800+ Megabytes of storage is overkill, another choice is required. Even at $50/Mbyte, a 404 Megabyte drive is not cheap. One of the authors has had good experience on a small 750 system with a 160 Mbyte Winchester disk drive from Tecstore and a National Semiconductor HEX-3000 combination tape and disk controller. We also know of successful use of the Spectra Logic combination controller on a 730 system. Using slightly less expensive disk drives and a combination controller one can obtain cost effective (< $75.00/Mbyte) storage in smaller amounts and provide a tape interface to boot (so to speak.)