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Top Forums UNIX for Advanced & Expert Users Physical disk IO size smaller than fragment block filesystem size ? Post 302770619 by rarino2 on Sunday 17th of February 2013 01:21:40 PM
Old 02-17-2013
Hello Praveen,
I'm not sure if you are mixing FS layer and phisical (disk driver) layer. I was speaking about FS layer (and block device driver layer) in my first post.

mcnamara answer was right.

Quote:
Originally Posted by Praveen_218
A block is still (on most FS) refers to 4K of data only.

On most system the page-cache is still of this fix sizes. Hence on all block I/O is of 4K or less. The file systems on such system utilizes the maximum size which is 4k.

However, if you see the disk architecture, they have been under trial by various vendors with various sector sizes; with 512bytes sector disk supported by most of the File system and storage product vendors (they however support various other size disks too -but 512b sector disk is in most common use probably because windows/DOS/UNIX FS supported them).

In order to support various disk architecture and FS supporting them use fragmentation which of course let you divide the 4k of page size into various fragments of 1, 2, ... 8 fragments per page.

8-fragments per page is the lowest value which translate into the size of a sector. You can of course not use a sector half of it. For I/O of 1 to full 512 byte of data a full sector gets used in one disk write.

Can you post here the steps you used to test this figures :
1) On UFS, how you saw the fragment size of 1 KB?
2) How did you looked at the 512kb I/O ?
1) fstype
2) dtrace script from DTrace Toolkit (bitesize.d)
 

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NEWFS_LFS(8)						    BSD System Manager's Manual 					      NEWFS_LFS(8)

NAME
newfs_lfs -- construct a new LFS file system SYNOPSIS
newfs_lfs [newfs_lfs-options] special DESCRIPTION
newfs_lfs builds a log-structured file system on the specified special device basing its defaults on the information in the disk label. Before running newfs_lfs the disk must be labeled using disklabel(8), the proper fstype is 4.4LFS. Reasonable values for the fsize, bsize, and sgs fields are 1024, 8192, and 7 respectively. The following options define the general layout policies. -A Attempt to compute the appropriate segment size using the formula 4 * bandwidth * access time. The disk is tested for twenty seconds to discover its bandwidth and seek time. -B logical-segment-size The logical segment size of the file system in bytes. If not specified, the segment size is computed by left-shifting the parti- tion label's block size by the amount indicated in the partition table's segshift. If the disklabel indicates a zero block size or segment shift, a compile-time default segment size of 1M is used. -b block-size The block size of the file system in bytes. If not specified, the block size is taken from the partition label, or if the parti- tion label indicates 0, a compile-time default of 8K is used. -F Force creation of an LFS even on a partition labeled as another type. newfs_lfs will use compile-time default values for block and fragment size, and segment shift, unless these are overridden by command-line flags. -f fragment-size The fragment size of the file system in bytes. If not specified, the fragment size is taken from the partition label, or if the partition label indicates 0, a compile-time default of 1K is used. -I interleave Specify the interleave between segments. The default is zero. -i The size of an inode block, in bytes. The default is to use the same size as a fragment, or in a v1 filesystem, the same size as a data block. -L Create a log-structured file system (LFS). This is the default, and this option is provided for compatibility only. -M nsegs Specify lfs_minfreeseg, the number of segments left out of the amount allocated to user data. A higher number increases cleaner performance, while a lower number gives more usable space. The default is based on the size of the filesystem, either 5% of the total number of segments or 20 segments, whichever is larger. -m free space % The percentage of space reserved from normal users; the minimum free space threshold. The default value used is 10%. -N Do not actually create the filesystem. -O offset Start the first segment this many sectors from the beginning of the partition. The default is zero. -R nsegs Specify lfs_resvseg, the number of segments set aside for the exclusive use of the cleaner. A larger figure reduces the likeli- hood of running out of clean segments, but if lfs_resvseg is too close to lfs_minfreeseg, the cleaner will run without ceasing when the filesystem becomes close to full. The default is the larger of 15 or the quantity lfs_minfreeseg / 2 + 1 . -r ident For a v2 filesystem, specify the roll-forward identifier for the filesystem. This identifier, a 32-bit numeric quantity, should be different from that of any LFS that may previously have existed on the same disk. By default the identifier is chosen at ran- dom. -s size The size of the file system in sectors. -v version Make a filesystem with the specified disk layout version. Valid options are 1 or 2 (the default). Note, however, that LFS ver- sion 1 is deprecated. SEE ALSO
disktab(5), disklabel(8), diskpart(8), dumplfs(8) M. Seltzer, K. Bostic, M. McKusick, and C. Staelin, "An Implementation of a Log-Structured File System for UNIX", Proceedings of the Winter 1993 USENIX Conference, pp. 315-331, January 25-29, 1993. J. Matthews, D. Roselli, A. Costello, R. Wang, and T. Anderson, "Improving the Performance of Log-Structured File Systems with Adaptive Methods", Proceedings of the Sixteenth ACM SOSP, October 1997. HISTORY
A newlfs command appeared in 4.4BSD, and was renamed to newfs_lfs for NetBSD 1.4. BSD
July 12, 2001 BSD
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