solomon@chaos.utexas.edu (Thomas Solomon) (04/02/91)
I recently received some product information from Dell computers.
They claim to have a controller that they refer to as the "Dell
Drive Array" that can handle up to 10 disk drives at the same time?
I get the impression from their information that this controller can
actually write to these different disks _at the same time_, allowing
blazing fast disk access.
Does anyone know about these drive arrays or about anyone elses
drive array. Presumably, with multiple disks operating simultaneously,
you should be able to get _continuous_ disk storage rates up to a
few MB/sec.
Tom Solomon
solomon@chaos.utexas.edugkendall@ncr-mpd.FtCollins.NCR.COM (Guy Kendall) (04/03/91)
In article <46563@ut-emx.uucp> solomon@chaos.utexas.edu (Thomas Solomon) writes: > Does anyone know about these drive arrays or about anyone elses >drive array. Presumably, with multiple disks operating simultaneously, >you should be able to get _continuous_ disk storage rates up to a >few MB/sec. NCR has recently introduced a SCSI-2 Disk Array board and a chip set. I don't have a lot of information about it, but if you would like more information I suggest calling Walt Krysiak at 719-596-5795. -- Guy W. Kendall - Applications Engineer Guy.Kendall@ColoSpgs.NCR.COM NCR Microelectronic Products Division Voice: 719-596-5795 x465 1635 Aeroplaza Drive Fax: 719-570-6045 Colorado Springs, CO 80916 BBS: 719-596-1649
marshall@wind55.seri.gov (Marshall L. Buhl) (04/09/91)
solomon@chaos.utexas.edu (Thomas Solomon) writes: > I recently received some product information from Dell computers. >They claim to have a controller that they refer to as the "Dell >Drive Array" that can handle up to 10 disk drives at the same time? >I get the impression from their information that this controller can >actually write to these different disks _at the same time_, allowing >blazing fast disk access. > Does anyone know about these drive arrays or about anyone elses >drive array. Presumably, with multiple disks operating simultaneously, >you should be able to get _continuous_ disk storage rates up to a >few MB/sec. I have looked at both the Dell drive array and the one that is an option on the Compaq SystemPro. They have several similarities. Both can "data stripe" to multiple drives simultaneously. Your data is broken up and stored on several drives. This means you're not limited to the transfer rate of a single drive. You're limited to the transfer rate of the EISA bus which is 32-bits wide. Both provide data guarding. On the Dell drive array, your data is striped on four disks with a fifth for parity. Thus, if one of your drives fails, the others will still contain all the data. The system will continue to run, but slower. When you get a replacement disk, take down the system and install it. The data will be restored to it from the other disks in the background while the machine goes back to its normal business. I think they can also provide disk mirroring, where two copies of your data are stored on different drives. I believe the differences are in performance. The Dell uses an i960 RISC processor and the Compaq uses an old 80186. IMO, this is the best argument I've seen for an EISA bus. I've heard that Dell has just started shipping their array, but in very small quantities. I was told that this was so that their support techs could get trained for it before mass shipping starts. From my Dell 1991 catalog: i960 @ 16 MHz 32-bit EISA interface 256K SRAM for local data storage i82355 bus master interface chip Up to 10 drives 33 MB/sec burst transfer rate SCSI drives are 200MB IDE drives w/ 16 msec average seek (Conner) works with DOS, OS/2, Unix, Vines and NetWare You will need an auxiliary power supply for more than 7 drives on the Dell. This will change after Dell uses up all their older power supplies and bigger ones are installed as standard equipment on their tower systems. -- Marshall L. Buhl, Jr. EMAIL: marshall@seri.gov Senior Computer Engineer VOICE: (303)231-1014 Wind Program 1617 Cole Blvd., Golden, CO 80401-3393 Solar Energy Research Institute Solar - safe energy for a healthy future
mmshah@athena.mit.edu (Milan M Shah) (04/10/91)
>solomon@chaos.utexas.edu (Thomas Solomon) writes: > > >> I recently received some product information from Dell computers. >>They claim to have a controller that they refer to as the "Dell >>Drive Array" that can handle up to 10 disk drives at the same time? >>I get the impression from their information that this controller can >>actually write to these different disks _at the same time_, allowing >>blazing fast disk access. > >Both can "data stripe" to multiple drives simultaneously. Your data is >broken up and stored on several drives. This means you're not limited >to the transfer rate of a single drive. You're limited to the transfer >rate of the EISA bus which is 32-bits wide. > >Both provide data guarding. On the Dell drive array, your data is >striped on four disks with a fifth for parity. All this should be taken with a grain of salt. First of all, it is not correct to expect that with n drives, your I/O system becomes n times faster than with 1 drive. It seems that the Dell drive uses the fifth drive for parity. This means that every write must write to the parity drive, so you are still limited by the data transfer rate of a single drive. So, even if sector 1 and sector 2 reside on different drives, I must access them sequentially cause I have to access the parity drive for each. Actually, a better idea is to spread the parity across all drives. ie, parity for sector 1 is stored on disk 1, for 2 on 2 for 6 back to 1 etc. Even then you are limited. Lastly, if you break up the time involved in a typical I/O request, one finds that about 8ms go for seek, about that much on rotating the disk so that the sector is under the head, and only about 2ms in actually transferring the data. With a single drive, on average you will have to wait for half a rotation to find your sector; with n drives, you tend to have to wait for one entire rotation (unless the drives are synchronized). You might actually loose here! This stuff is actually enough for a Master's thesis (mine, I hope!!!) and therefore can't be covered in any detail here. For an excellent background, read David Patterson's and Katz's papers on RAID - Redundant arrays of inexpensive disks. I believe that to a very crude first approximation, the merits of drive arrays lie in the fact that a 2 gigabyte store made up of 10 200 Meg drives will be cheaper, more reliable and *maybe* faster than a single 2 Gigabyte drive. It is incorrect to assume that a 2 gigabyte store made up of 10 200 Meg drives will be 10 times faster than a single 200 Meg drive. But of course, I am ready to bet every last dollar I have that the marketing types at Dell and Compaq and IBM etc., will convince one and all that in fact their arrays are 10 times faster, and will sell a ton load of them, leaving a trail of puzzled and appropriately impoverished customers behind. Hey, if 4 times oversampling in CD players is such a good thing, drive arrays of 10 gotta be good too, eh? ;-) Milan .
rick@always.com (Rick Wagner) (04/12/91)
In article <1991Apr9.210740.16385@athena.mit.edu> mmshah@athena.mit.edu (Milan M Shah) writes: >>solomon@chaos.utexas.edu (Thomas Solomon) writes: >>Both provide data guarding. On the Dell drive array, your data is >>striped on four disks with a fifth for parity. > >Actually, a better idea is to spread the parity across all drives. ie, parity >for sector 1 is stored on disk 1, for 2 on 2 for 6 back to 1 etc. Even then >you are limited. > The fifth drive is probably not parity, but ECC, allowing for correction of lost data. That way, if one drive goes bad, (think of the array as four data drives, and one ECC drive), if its a data drive, then the ECC corrects the lost data bytes. If it's the ECC drive, then you continue running with just the data. When you get a replacement, the system should reconstruct the new drive from the existing data. Not saying thats what the Dell or Compaq DO, but thats the way I understand some arrays to work. --rick -- =============================================================================== Rick Wagner email: rick@always.COM Always Technology Voice: 1-818-597-1400 31336 Via Colinas, Suite 101 Fax: 1-818-597-1496