Date: Mon, 03 Jul 2000 18:58:13 -0700 From: Mike Smith <msmith@freebsd.org> To: David Gilbert <dgilbert@velocet.ca> Cc: freebsd-stable@freebsd.org Subject: Re: Dump takes ages on SCSI. (several times faster on IDE) Message-ID: <200007040158.SAA00532@mass.osd.bsdi.com> In-Reply-To: Your message of "Sun, 02 Jul 2000 16:25:14 EDT." <14687.42282.873585.2561@trooper.velocet.net>
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> I have a PIII/450 with a venerable Adaptec 2940UW and a Quantum 9G > SCSI disk. > > Recenly, I was generating many crash dumps and was using a serial > console (so I was watching them). I came to a point in my debugging > where I needed another PCI slot, so I removed the 2940 and loaded an > IDE drive with the same build of FreeBSD. > > I then noticed that the IDE would crashdump somewhere around 10 times > faster than the SCSI disk. > > Why? Can the SCSI speed be improved? IDE disks typically default to write-cache enabled (the drive returns the command as complete as soon as it's given to the drive), while SCSI disks default to write-cache disabled (the drive doesn't return the command as complete until it's hit the media). In the dump context, this means that with a SCSI disk you spend a lot of time waiting for this back-and-forth handshaking, which you don't get with IDE disks. If you have the WCE bit set on the drive in question, it will go faster. The fact that SCSI commands are more expensive than IDE commands is also significant given that dump I/O is performed one page at a time. -- \\ Give a man a fish, and you feed him for a day. \\ Mike Smith \\ Tell him he should learn how to fish himself, \\ msmith@freebsd.org \\ and he'll hate you for a lifetime. \\ msmith@cdrom.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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