Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!wuarchive!zaphod.mps.ohio-state.edu!think.com!mintaka!bloom-beacon!eru!hagbard!sunic!mcsun!ukc!mucs!logitek!sph From: sph@logitek.co.uk (Stephen Hope) Newsgroups: comp.periphs.scsi Subject: Re: What makes a SCSI drive fast? Keywords: SCSI drive Message-ID: Date: 18 Apr 91 16:33:50 GMT References: <14971@life.ai.mit.edu> Organization: Logitek Plc. Lines: 35 fur@ai.mit.edu (Scott Furman) writes: >I have seen a lot of info posted in this group about SCSI transfer rates. >However, I have not seen as many postings about other performance parameters of >SCSI drives. >Recently I was reading some SCSI spec sheets for the Wren IV. A few parameters >caught my eye: >1) Overhead time for head switch (512 byte sectors): < 2 ms >2) Overhead time for one-track cyclinder switch : 6 ms typical >3) Average rotational latency : 8.3 ms >Two milliseconds to switch heads!? I would have guessed that switching sense >heads was was done purely by electrical means. The overhead time for a track >change also seems excessive. Apparently these overhead times are due to the use >of embedded servo mechanisms which are commonly found now in modern drives. >Embedded servos, I am told, increase reliability and reduce the price per byte. >Does anyone have an explanation of why embedded servo are more reliable? Embedded servo means that the drive uses positioning info for track following embedded in the track data. This is superior to a separate servo surface, since head or spindle misalignment or thermal effects cannot cause positioning errors. This in turn allows tracks to be closer together, increasing drive capacity. >-Scott Stephen Hope #include