Date: Sat, 25 Apr 1998 15:04:09 -0700 From: Matthew Jacob <mjacob@feral.com> To: louie@TransSys.COM, mjacob@feral.com Cc: dkelly@HiWAAY.net, freebsd-scsi@FreeBSD.ORG, Harlan.Stenn@pfcs.com Subject: Re: does CAM do this? Message-ID: <199804252204.PAA28488@feral.com>
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>From louie@whizzo.TransSys.COM Sat Apr 25 15:02:44 1998 >> If, and only if, the tape is in variable block mode. If it's >> in fixed block mode, and the tape driver hasn't set SILI (Suppress >> Incorrect Length Indicator) you get an error if the requested >> byte count doesn't match the tape block size. > >Well, sure, but if you don't know what the blocksize is, why would you >be using fixed block mode? My impression is that fixed block mode is >usually used for devices in which the blocksize is, well, fixed. That is, >either the media or drive mechanism don't support arbitrary or variable >record sizes. I understand that some QIC tape systems are like this. > >Other media, such as 9 track tapes, or Exabyte 8mm or DAT 4mm media >give you variable sized records. From my personal experience, DAT >drives also support fixed block mode, but I'm not really sure why you >would use it unless it's a compatiblity hack for software which might >presume a (e.g., QIC) fixed blocksize media. > >In reality, the low-level format on the tape consists of a set of fixed length >physical helical swipes, with a directory which points to the location of >the logical tape records written to the drive. This is the reason why >there's not a significant difference in performance or capacity between >writing 512 byte blocks or 20480 byte blocks; the difference is only >in the pointers to the logical records recorded on the tape. > >So, if you have the possibility of variable sized blocks on the tape >media, why would use fixed block mode in accessing the drive? Some drives- including Exabytes- can come up in fixed block mode as the default. It's a property of the drive, not the tape. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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