Date: Sat, 25 Apr 1998 18:02:41 -0400 From: "Louis A. Mamakos" <louie@TransSys.COM> To: Matthew Jacob <mjacob@feral.com> Cc: Harlan.Stenn@pfcs.com, dkelly@HiWAAY.net, freebsd-scsi@FreeBSD.ORG Subject: Re: does CAM do this? Message-ID: <199804252202.SAA00591@whizzo.TransSys.COM> In-Reply-To: Your message of "Sat, 25 Apr 1998 13:00:11 PDT." <199804252000.NAA28239@feral.com> References: <199804252000.NAA28239@feral.com>
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> 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? louie To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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