Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!fed!m1rcd00 From: m1rcd00@fed.frb.gov (Bob Drzyzgula) Newsgroups: comp.periphs Subject: Re: Disk Performance Summary: Summary and new questions Keywords: SCSI IPI Message-ID: <939@arccs2.fed.FRB.GOV> Date: 5 Feb 91 18:36:50 GMT References: <934@arccs2.fed.FRB.GOV> Sender: news@fed.FRB.GOV Reply-To: rcd@arccs3.UUCP (Bob Drzyzgula) Organization: Federal Reserve Board, Wash, DC Lines: 161 Thank you to everyone who sent replies to my question. I had substantive replies from: uunet!twinsun.com!eggert (Paul Eggert) uunet!cs.UAlberta.CA!steve (Steve Sutphen) uunet!xstor!billbr (Bill Brothers) David Clapp While these were interesting contributions, I fear they raised as many, or perhaps more, questions than they answered. Here are some of these: * Has anyone done a comparative benchmark of systems similarly configured except for disk? * What can one expect transfer rates (disk platter to host memory) to be for SCSI drives in a UNIX (esp. Sun) environment? Bill Brothers points out that this may well be significantly less than the 5MB/Sec quoted for many drives. Seagate Elite drives quote an internal transfer rates of 24 to 32 Mbits/sec (3-4 MB/sec if these numbers don't include any overhead. Older drives such as the Wren 5 have rates around 9-15 Mbits/sec). Does this give any clue to the actual transfer rates obtainable with these devices? What is the comparable number for IPI drives of various speeds? If two drives are the same internally but have SCSI and IPI interfaces, and both quote the same internal xfr rate, and the SCSI drive has an external xfr rate quoted that is significantly above the internal rate, then can one assume that these drives will perform equivalently? * What is the state of SCSI-to-VME Host adapter performance? * Drive arrays are interesting creatures. The Ciprico 6600 (I think that's the number) parallel disk array controller can potentially be used to stripe across four 5.25" drives, keeping a fifth one with redundant information so's you can rebuild any drive that craps out (if it does so before the controller dies, I presume) Two questions: Has anyone seen a product made out of this thing? That you can buy? It's been around for over a year now. And what is the implication of this being a SCSI-2 device, given the limitations of SCSI cited? Can SCSI-2 really crank at 20 MB/Sec? On a UNIX machine? And, another couple of questions and comments: * What are the pros and cons of synchronous SCSI-2? Are there any reasons *not* to use it? * Believe it or not, disk drive capacities are now getting to a point that they really are worrying me. Drive manufacturers seem to be pushing in the directions of speed and capacity, both at the same time. If you want faster drives, you gotta buy the bigger ones. I don't know about you, but for me, 3GB gets to be a little unwieldy, especially when you can only carve it up into seven or eight chunks. And when one drive takes care of the entire storage requirements of a machine, how do you take advantage of multiple disk arms? Don't you run the risk of driving up the real-use average seek time if you have to spread heavily used filesystems all over a single drive? I know that these drives are quite a bit faster, but running them in parallel should be faster even yet. * Are there any signs of IPI-3 intelligent controllers for Suns on the horizon? * Re Paul Eggert's comment on MTBFs: Seagate quotes 150,000 Hrs for the Elite drives, and 250,000 hours in a Class A computer room. 250,000 Hrs is about 28.5 years. Imagine the drum on your IBM 360/20 breaking now for the first time... * And a solicitation for further comments on these issues... Thanks again, Bob Drzyzgula Federal Reserve Board rcd@fed.frb.gov ================================================================== Herewith are the replies: Paul Eggert had the following comments: ----------- My first advice is to *benchmark the drives using your application*. It's OK to buy blind for toy applications, but if you're contemplating a big purchase there's no excuse for buying without trying first. Manufacturers' spec sheets tell you performance under ideal conditions, not real conditions. For the drives you list, don't forget to add the average latency to the average seek time. On typical 3600 RPM drives this is 8.33ms; it's simple arithmetic of figuring out how long half a rotation talks. I believe the Elite-1s rotate at 5400 rpm, so their average access time is reduced. My experience is that the prices on these big SCSI drives have fallen dramatically quite recently -- if you can get them. Frankly, for most applications, I don't see how IPI can be worth the big price differential. Just buy more SCSI drives and do your application in parallel. The big question is: how reliable are the drives? In the past this has been a dominant factor in my decision making, but I'm wondering now how true it is, anymore. Most drive vendors are quoting >=100,000 hours (11 years) MTBF. This means it's far more probable for your electronics to fry, (or your drives to become obsolete!), than for them to fail. If only we had MTBF figures for entire subsystems! If you want speed, you might look into the Maxtor P1-12S: average seek time is 10.5ms, latency 8.33ms, reasonable price. I haven't used one. Steve Sutphen added: ------------- I don't have `THE' answer to your question, but did want to point out one thing. SCSI is a two step process getting the data from the disk head to the controller, while IPI-2 is a one-step as was SMD (this is a simplification). What happens in SCSI drives is that the data is read off the disk and put in a local memory. This memory is then transferred over the SCSI bus to the host. When SCSI transfer rates are quoted they are the maximum clock rate that data can be transferred from the local memory (a cache) to the host--it bears no relationship to the sustainable throughput (i.e. how many MB/sec can one read on large transfers). This is not to say that the Elite SCSI system is not cost effective for a lot of applications, I am sure that they are. From what I have read they are a wonderful drive (although they are new and unproven for reliability). The only real way to find out is to do testing (a hard process, maybe someone else has done some recent tests). From Bill Brothers, ------------- In response to your questions about large drives and controller performance... There is no easy answer. Perhaps one thing that you didn't mention is transactions per second. The number of transactions a given unit can handle dramatically varies from manufacturer to manufacturer, and so should be considered. This is directly related to access time. The biggest problem I see with your analysis fails to take into consideration the actual operating characteristics of the host adapter (for SCSI) or the controller (for IPI-2). Being able to dump data at 5 Mbytes per second off of the drive doesn't mean that your host adapter or bus can take it at that rate. The other part of this is that 5 Mbytes/s is when you issue requests to the drive in chunks of 64K or more. Standard UNIX requests of 1,2,4, or 8K will not be able to achieve that data rate. IPI-2 is sort-of like super ESDI. It has very little overhead and can handle high transfer rates. Thus, if your machine can actually deal with the data rates involved, IPI-2 performs much better. Then there is the issue of cost... All in all, it is a complex question that has no simple answer. I would be interested in what other people have to say about it. And from David Clapp, ----------- One other approach to consider is disk arrays. They've been rumored for years and seem to be available at last. I've seen (no experience!) add-on systems from ciprico and bundled offerings from NCR, for example. This promises to combine the cheap medium performance drives into arrays that have redundancy and improved performance. ==================================================================