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Top Forums UNIX for Advanced & Expert Users dd bytesize Post 302515137 by programAngel on Tuesday 19th of April 2011 08:00:10 AM
Old 04-19-2011
dd bytesize

I read the manual about the dd and the example and I have seen the use of the bs option, the byte size.

However how can you know that you need to use it and how can you know what size to give it?

like in wwikipedia they give the following:
Quote:
Using dd to duplicate one hard disk partition to another hard disk:
dd if=/dev/sda2 of=/dev/sdb2 bs=4096 conv=noerror
but why 4096 and why mention it at all
 
jfs_mkfs(8)						     JFS file system creation						       jfs_mkfs(8)

NAME
jfs_mkfs - create a JFS formatted partition SYNOPSIS
jfs_mkfs [options] device [ blocks ] DESCRIPTION
jfs_mkfs is used to create (format) a JFS partition. jfs_mkfs must be run as root. device is the special file name corresponding to the actual device (e.g. /dev/hdb1) on which a JFS file system and/or JFS journal will be created. blocks is the number of blocks to be used for the file system. If omitted, jfs_mkfs automatically figures the file system size. The default block size is 4096. WARNING
jfs_mkfs will destroy all data on the specified device! OPTIONS
-c Check the device for bad blocks before building the file system. -j journal_device Create the external JFS journal on journal_device , create the JFS file system on device , and attach the external journal to the file system. -J journal-options Create the JFS journal on its own device or attach an external JFS journal to the JFS file system using options specified on the command-line. The following journal options are supported: journal_dev Create an external JFS journal on the given device. device=external-journal Attach an existing external JFS journal located on external-journal to the JFS file system that will be created on device. The external journal must already have been created using the command jfs_mkfs -J journal_dev external-journal Attach the external journal to the file system being created by using the command jfs_mkfs -J device=external-journal device Instead of specifying a device name directly, external-journal can also be specified by either LABEL=label or UUID=UUID to locate the external journal by either the volume label or UUID stored in the JFS external log superblock at the start of the journal. Use jfs_tune(8) to display a journal device's volume label and UUID. (See the -l, -L, and -U options of jfs_tune(8).) Only one of the journal_dev or device= options can be given per jfs_mkfs issuance. -L volume_label Set the volume label for the newly formatted device. If -L volume_label is used along with -J journal_device, the volume label of the specified external journal device will be set to volume_label. Otherwise, the volume label of the JFS file system will be set to volume_label. -O Provide case-insensitive support for compatibility with OS/2. -q Quiet execution - do not ask for confirmation before creating the file system. -s log_size Set the log size (in megabytes) of the inline log. If the -s option is not used, the default log size is 0.4% of the aggregate size. -V Print version information and exit (regardless of any other chosen options). EXAMPLES
Format 3rd partition on 2nd hard disk with the volume label "JFS_hdb3": jfs_mkfs -L JFS_hdb3 /dev/hdb3 Format 5th partition on 1st hard disk, verifying each block created: jfs_mkfs -c /dev/hda5 Format 3rd partition on 2nd hard disk as external journal, format 4th partition on 1st hard disk as JFS file system, and attach the exter- nal journal to the file system: jfs_mkfs -j /dev/hdb3 /dev/hda4 REPORTING BUGS
If you find a bug in JFS or jfs_mkfs, please report it via the bug tracking system ("Report Bugs" section) of the JFS project web site: http://jfs.sourceforge.net/ Please send as much pertinent information as possible including any error messages resulting from running jfs_mkfs. SEE ALSO
mkfs(8), jfs_fsck(8), jfs_fscklog(8), jfs_tune(8), jfs_logdump(8), jfs_debugfs(8) AUTHORS
Barry Arndt (barndt@us.ibm.com) William Braswell, Jr. jfs_mkfs is maintained by IBM. See the JFS project web site for more details: http://jfs.sourceforge.net/ March 9, 2005 jfs_mkfs(8)
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