Date: Fri, 9 Feb 2018 22:23:10 +0000 (UTC) From: Kirk McKusick <mckusick@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r329087 - head/tools/diag/prtblknos Message-ID: <201802092223.w19MNAr1021793@repo.freebsd.org>
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Author: mckusick Date: Fri Feb 9 22:23:10 2018 New Revision: 329087 URL: https://svnweb.freebsd.org/changeset/base/329087 Log: Add an explanation of the block size units output by prtblknos. Suggested by: Ravi Pokala (rpokala@) Modified: head/tools/diag/prtblknos/README Modified: head/tools/diag/prtblknos/README ============================================================================== --- head/tools/diag/prtblknos/README Fri Feb 9 21:51:08 2018 (r329086) +++ head/tools/diag/prtblknos/README Fri Feb 9 22:23:10 2018 (r329087) @@ -27,5 +27,22 @@ The distance metric is the size of the gap from the en previous set of blocks to the beginning of the next set of blocks. Short distances are desirable. +The logical block numbers (lbn above) describe filesystem-block +size blocks which by today's default is 32Kb. + +The physical block numbers (blkno above) describe the smallest piece +of disk space that an inode can reference which is a filesystem +fragment. Since the default size for fragments today is 4Kb, the +physical block numbers reference 4Kb blocks. The distances listed +above are also in physical block size units. Thus a distance of 8 +means a separation of just one 32Kb block. For example lbn 11, ends +at 3217895, the 32Kb first level indirect immediately follows it at +3217896-3217903, and lbn 12 starts distance 8 after lbn 11 immediately +after the indirect block at 3217904. + +If you were to create a filesystem with the fragment size equal to +half the block size (say 32Kb blocks with 16Kb fragments) then the +physical block numbers listed would be 16Kb in size. + Marshall Kirk McKusick January 19, 2018
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