Date: Tue, 4 Jan 2000 11:36:13 -0500 (EST) From: Jim Doherty <doherty@ans.net> To: Steve Passe <smp@csn.net> Cc: Bruce Evans <bde@zeta.org.au>, Peter Jeremy <peter.jeremy@ALCATEL.COM.AU>, hardware@FreeBSD.ORG Subject: Re: wanna buy an EIDE harddisk ... 5400 or 7200 for home use (noise) Message-ID: <Pine.GSO.4.05.10001041131310.5846-100000@sandcastle.ny.ans.net> In-Reply-To: <200001041622.JAA41318@Ilsa.StevesCafe.com>
next in thread | previous in thread | raw e-mail | index | archive | help
If your interested in general storage and/or scsi issues comp.arch.storage FAQ and comp.periphs.scsi FAQ cover them fairly well. from comp.periphs.scsi FAQ: Should I spend the extra money on SCSI or just get IDE? ANSWER From: Gary Field (gfield@zk3.dec.com) ==== For home users this is a difficult question to answer in general. It totally depends on how you use your system, what operating system is installed, and whether you will add more I/O devices in the future. For server systems in a corporate environment the only sensible answer is to go with SCSI peripherals. IDE/EIDE is single threaded by nature. The current command must complete before additional commands can start. With most IDE adapters the processor must be involved in reading/writing the data from/to memory. Another drawback is that only two drives can be attached. In a single drive single-tasking system IDE will probably be slightly faster and is definitely less expensive. When you start talking about multi-tasking operating systems (like Win95, WinNT, Unix, OS/2 and Netware) SCSI is now a big advantage. As disk drives get bigger, backup devices are becoming even more important. In my opinion floppy tapes just aren't satisfactory. They're too slow, too unreliable, non-portable(media exchange wise not physically), and have low storage capacities. SCSI tape drives are more expensive, but have none of these problems. SCSI devices share the bus bandwidth efficiently by allowing one device to transfer data while another is seeking or rewinding its media. Early SCSI implimentations had some compatibility problems but these days SCSI is simpler to install than EIDE. Each user needs to make this choice individually, but if you don't consider all the issues, you can find yourself needing to re-vamp all your I/O to add a device later on. Before you decide to go with IDE, ask yourself if you will ever want to add a CDROM, CD-R, scanner, or tape drive or need more than two hard disk drives. and in a little more detail: Here's a discussion that shows some of the advantages of SCSI in more detail: from: Greg Smith (GREGS@lss-chq.mhs.compuserve.com) Under DOS (and DOS/win3.1), there is very little useful work the host can do while waiting for a disk operation to complete. So handing off some work from a 66 MHz 486 to, say, an 8 MHz Z80 (on the controller) does result in a performance loss. Under EVERY other OS worth discussing (Unix, Netware, NT, OS/2, Win95 etc) the processor can go off and do something else while the access is in progress, so the work done by the other CPU can result in a performance increase. In such systems, due to virtual memory, a 64K byte 'contiguous' read requested by a process may be spread to 16 separate physical pages. A good SCSI controller, given a single request, can perform this 'scatter/gather' operation autonomously. ATA requires significant interrupt service overhead from the host to handle this. Another big issue: ATA does not allow more than one I/O request to be outstanding on a single cable, even to different drives. SCSI allows multiple I/O requests to be outstanding, and they may be completed out of order. For instance, process 'A' needs to read a block. The request is sent to the drive, the disk head starts to move, and process 'A' blocks waiting for it. Then, process 'B' is allowed to run; it aslo reads a block from the disk. Process B's block may be sitting in a RAM cache on the SCSI controller, or on the drive itself. Or the block may be closer to the head than process A's block, or on a different drive on the same cable. SCSI allows process B's request to be completed ahead of process A's, which means that process B can be running sooner, so that the most expensive chip - the system CPU - tends to spend less time twiddling its thumbs. Under ATA, the process B request cannot even be sent to the drive until the process A request is complete. These SCSI capabilities are very valuable in a true multi-tasking environment, especialy important in a busy file server, and useless under DOS, which cannot take advantage of them. I tend to hear from people, 'Well, I never use multitasking' because they never actively run two programs at once - all but one are 'just sitting there'. Consider what happens though, when you minimize a window which uncovers parts of four other application windows. Each of those applications is sent a message telling it to update part of its window; under win95, they will all run concurrently to perform the update. If they need to access disk (usually because of virtual memory) the smoothness of the update can depend a lot on the disk system's ability to respond to multiple independent read requests and finish them all as quickly as possible; SCSI is better at this. So, yes, ATA is faster under DOS; but SCSI provides advantages which are inaccessible to DOS. They will benefit Win95 however. The cost of intelligent, fast SCSI controllers and drives should decrease as people discover these advantages and start buying them. I should add that many of SCSI's advantages are NOT available with some of the simpler SCSI controllers which were targeted only to the DOS market or part of cheap CDROM add-on kits. Furthermore, SCSI allows far greater flexibility of interconnect. I concede that for the mass market, which likes to buy pre-configured machines, this is but a small advantage. Jim Doherty To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hardware" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?Pine.GSO.4.05.10001041131310.5846-100000>