Start copying large file while its still being restored from tape


 
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Old 07-15-2011
I'd still recommend some way to verify the entire file made it across, like an md5 sum on both sides.

Read-behind-write is fraught with lots of chances to not get the entire file. You pretty much have to know that your entire software stack is set up properly. Even in the sleep-then-cat example, and even if the network is supposedly guaranteed to be slower than the tape, any hiccup reading the tape - or in the data path from tape to disk - could allow the cat to catch up with the file end.

Never mind what happens if you run into a restore process that sets the file length prior to actually writing out the data....
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MT(1)							      General Commands Manual							     MT(1)

NAME
mt - magnetic tape manipulating program SYNOPSIS
mt [ -f tapename ] command [ count ] DESCRIPTION
Mt is used to give commands to a magnetic tape drive. If a tape name is not specified, the environment variable TAPE is used; if TAPE does not exist, mt uses the device /dev/rmt12. Note that tapename must reference a raw (not block) tape device. By default mt performs the requested operation once. Operations may be performed multiple times by specifying count. The available commands are listed below. Only as many characters as are required to uniquely identify a command need be specified. eof, weof Write count end-of-file marks at the current position on the tape. fsf Forward space count files. fsr Forward space count records. bsf Back space count files. bsr Back space count records. rewind Rewind the tape (Count is ignored). offline, rewoffl Rewind the tape and place the tape unit off-line (Count is ignored). status Print status information about the tape unit. cacheon Enable the readahead/writebehind cache on the tape unit. cacheoff Turn off the readahead/writebehind cache on the tape unit. Mt returns a 0 exit status when the operation(s) were successful, 1 if the command was unrecognized, and 2 if an operation failed. The cacheon and cacheoff commands currently only apply to the TMSCP driver and then only for certain drives such as the TU81+. No error is produced by the TMSCP driver if the cache commands are used. Other drivers will return an error code since they do not recognize the MTCACHE and MTNOCACHE functions codes. See mtio(4). FILES
/dev/rmt* Raw magnetic tape interface SEE ALSO
mtio(4), tmscp(4), dd(1), ioctl(2), environ(7) 4th Berkeley Distribution December 22, 1995 MT(1)