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Full Discussion: How do I determine the best number to use for the bs (block size) operand of the dd command?
Top Forums UNIX for Beginners Questions & Answers How do I determine the best number to use for the bs (block size) operand of the dd command? Post 303011890 by Quenz on Thursday 25th of January 2018 05:28:23 AM
Old 01-25-2018
How do I determine the best number to use for the bs (block size) operand of the dd command?
When I create a bootable Linux distro installation USB drive, I use this command:
Code:
sudo dd if=/Path/to/linux_distro.iso of=/dev/rdisk<disk number>
 bs=<number of bytes>

When I look it up, I've seen variations of people choosing 4M, and I think 8M, 2M, and maybe even 1M.

If I leave the operand off, it seems to take a long time (I never let it finish).

How would I determine what number to use to make it go the fastest? Is speed the only factor?
 

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LEARN ABOUT ULTRIX

tz

tz(4)							     Kernel Interfaces Manual							     tz(4)

Name
       tz - SCSI magnetic tape interface

Syntax
       VAX NCR 5380:
	 adapter      uba0    at nexus?
	 controller   scsi0   at uba0	 csr 0x200c0080  vector szintr
	 tape	      tz0     at scsi0	 drive 0

       VAX DEC SII:
	 adapter      ibus0   at nexus?
	 controller   sii0    at ibus?	 vector sii_intr
	 tape	      tz0     at sii0	 drive 0

       RISC DEC SII:
	 adapter      ibus0   at nexus?
	 controller   sii0    at ibus?	 vector sii_intr
	 tape	      tz0     at sii0	 drive 0

       RISC DEC KZQ:
	 adapter      uba0    at nexus?
	 controller   kzq0    at ibus? csr 0761300vector sii_intr
	 tape	      tz0     at kzq0	 drive 0

       RISC NCR ASC:
	 adapter      ibus0   at nexus?
	 controller   asc0    at ibus?	 vector ascintr
	 tape	      tz0     at asc0	 drive 0

Description
       The SCSI tape driver provides a standard tape drive interface as described in This is a driver for any Digital SCSI tape device.

       For  the  TZK10	QIC  format tape drive, the densities supported are QIC-24 (read only) block size of 512 byte blocks, QIC-120, and QIC-150
       read/write block size of 512 byte blocks, and QIC-320 read/write block size of 1024 byte blocks.  With QIC format style tapes all reads and
       writes  must  be  in  multiple  of the block size.  This is a requirement of fixed block tape drives because record boundaries are not pre-
       served.	The QIC densities are selected using the following special device names:

	 QIC-24 Fixed block size.
	 QIC-120 Fixed block size.
	 QIC-150 Fixed block size.
	 QIC-320 Fixed block size.

       With all fixed block tape devices a of a file to the tape must be padded out.  An example of this is a of which has a size of approximately
       3800 bytes.
       dd if=/etc/gettytab of=/dev/rmt0h bs=10k conv=sync
	 or
       dd if=/etc/gettytab of=/dev/rmt0l bs=512 conv=sync
       The option of pads the output to block size.

       This  driver  also  supports  n-buffered  reads	and  writes  to the raw tape interface (used with streaming tape drives).  See for further
       details.

Tape Support
       TZ30, TZK50, TLZ04, TSZ05, TKZ08, TZK10

Diagnostics
       All diagnostic messages are sent to the error logger subsystem.

Files
See Also
       mtio(4), nbuf(4), SCSI(4), MAKEDEV(8), uerf(8), tapex(8)
       Guide to the Error Logger

																	     tz(4)
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