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SNFSDEFRAG(1)                                                                                  SNFSDEFRAG(1)



NAME
       snfsdefrag - Xsan File System Defrag Utility

SYNOPSIS
       snfsdefrag [-DdPqsv] [-G group] [-K key] [-k key] [-m count] [-r] [-S file] Target[Target...]

       snfsdefrag -e [-b] [-G group] [-K key] [-r] [-t] [-S file] Target [Target...]

       snfsdefrag -E [-b] [-G group] [-K key] [-r] [-t] [-S file] Target [Target...]

       snfsdefrag -c [-G group] [-K key] [-r] [-t] [-S file] Target [Target...]

       snfsdefrag -p [-DvPq] [-G group] [-K key] [-m count] [-r] [-S file] Target [Target...]

       snfsdefrag -l [-Dv] [-G group] [-K key] [-m count] [-r] [-S file] Target [Target...]

DESCRIPTION
       snfsdefrag is a utility for defragmenting files on an Xsan volume by relocating the data in a file to
       a smaller set of extents.  Reducing the number of extents in a file improves performance by  minimiz-ing minimizing
       ing  disk  head movement when performing I/O.  In addition, with fewer extents, Xsan File System Man-ager Manager
       ager (FSM) overhead is reduced.

       By default, the new extents are created using the file's current storage pool affinity.  However, the
       file  can  be "moved" to a new storage pool by using the -k option.  This migration capability can be
       especially useful when a storage pool is going out of service.  See the use of the -G option  in  the
       EXAMPLES section below.

       In  addition  to  defragmenting  and migrating files, snfsdefrag can be used to list the extents in a
       file (see the -e option) or to prune away unused space that has been preallocated for the  file  (see
       the -p option).

OPTIONS
       -b     Show  extent  size  in  blocks  instead  of  kilobytes.   Only useful with the -e and -E (list
              extents) options.

       -c     This option causes snfsdefrag to just display an extent count instead of defragmenting  files.
              See also the -t option.

       -D     Turns on debug messages.

       -d     Causes  snfsdefrag  to operate on files containing extents that have depths that are different
              than the current depth for the extent's storage pool.  This option is  useful  for  reclaiming
              disk  space  that has become "shadowed" after cvupdatefs has been run for bandwidth expansion.
              Note that when -d is used, a file may be defragmented due to the stripe depth in one  or  more
              of its extents OR due to the file's extent count.

       -e     This  option causes snfsdefrag to not actually attempt the defragmentation, but instead report
              the list of extents contained in the file.  The extent information includes the starting  file
              relative offset, starting and ending storage pool block addresses, the size of the extent, the
              depth of the extent, and the storage pool number.  See also the -t option.

       -E     This option has the same effect as the -e option except that file relative offsets and  start-ing starting
              ing  and  ending  stripe group block addresses that are stripe-aligned are highlighted with an
              asterisk (*).  Also, starting storage pool addresses that are equally misaligned with the file
              relative  offset are highlighted with a plus sign (+).  Currently, this option is intended for
              use by support personnel only.  See also the -t option.

       -G storagepool
              This option causes snfsdefrag to only operate on files having at least one extent in the given
              storage pool.  Note that multiple -G options can be specified to match files with an extent in
              at least one of the specified storage pools.

       -K key This option causes snfsdefrag to only operate on source files that have the supplied  affinity
              key.   If key is preceded by '!' then snfsdefrag will only operate on source files that do not
              have the affinity key.  See EXAMPLES below.

       -k key Forces the new extent for the file to be created on the storage pool specified by key.

       -l     This option causes snfsdefrag to just list candidate files.

       -m count
              This option tells snfsdefrag to only operate on files containing more than count extents.   By
              default, the value of count is 1.

       -p     Causes  snfsdefrag  to  perform a prune operation instead of defragmenting the file.  During a
              prune operation, blocks beyond EOF that have been preallocated either explicitly or as part of
              inode expansion are freed, thereby reducing disk usage.  Files are otherwise unmodified. Note:
              While prune operations reclaim unused disk space, performing them regularly can lead  to  free
              space fragmentation.

       -P     Lists skipped files.

       -q     Causes snfsdefrag to be quiet.

       -r [TargetDirectory]
              This option instructs snfsdefrag to recurse through the TargetDirectory and attempt to defrag-ment defragment
              ment each fragmented file that it finds.  If TargetDirectory is  not  specified,  the  current
              directory is assumed.

       -s     Causes snfsdefrag perform allocations that line up on the beginning block modulus of the stor-age storage
              age pool. This can help performance in situations where the I/O size perfectly spans the width
              of the storage pool's disks.

       -S file
              Writes  status  monitoring  information in the supplied file.  This is used internally by Xsan
              and the format of this file may change.

       -t     This option adds totals to the output of the -c, -e, or -E options.  The one  line  output  at
              the  end indicates how many regular files were visited, how many total extents were found from
              all files, and the average # of extents per file.

       -v     Causes snfsdefrag to be verbose.

EXAMPLES
       Count the extents in the file foo.

          rock% snfsdefrag -r -c -t dir1

       Starting in directory, dir1, recursively count all the files and their extents  and  then  print  the
       grand total and average number of extents per file.

          rock% snfsdefrag -c foo

       List the extents in the file foo.

          rock% snfsdefrag -e foo

       Defragment the file foo.

          rock% snfsdefrag foo

       Defragment the file foo if it contains more than 2 extents.  Otherwise, do nothing.

          rock% snfsdefrag -m 2 foo

       Traverse  the  directory  abc and its sub-directories and defragment every file found containing more
       than one extent.

          rock% snfsdefrag -r abc

       Traverse the directory abc and its sub-directories and defragment every file found having one or more
       extents  whose  depth differs from the current depth of extent's storage pool OR having more than one
       extent.

          rock% snfsdefrag -rd abc

       Traverse the directory abc and its sub-directories and only  defragment  files  having  one  or  more
       extents whose depth differs from the current depth of extent's storage pool.

          rock% snfsdefrag -m 9999999999 -rd abc

       Traverse the directory abc and recover unused preallocated disk space in every file visited.

          rock% snfsdefrag -rp abc

       Force the file foo to be relocated to the storage pool with the affinity key "fast"

          rock% snfsdefrag -k fast -m 0 foo

       If the file foo has the affinity fast, then move its data to a storage pool with the affinity slow.

          rock% snfsdefrag -K fast -k slow -m 0 foo

       If the file foo does NOT have the affinity slow, then move its data to a storage pool with the affin-ity affinity
       ity slow.

          rock% snfsdefrag -K '!slow' -k slow -m 0 foo

       Traverse the directory abc and migrate any files containing at least one extent in storage pool 2  to
       any non-exclusive storage pool.

          rock% snfsdefrag -r -G 2 -m 0 abc

       Traverse  the directory abc and migrate any files containing at least one extent in storage pool 2 to
       storage pools with the affinity slow.

          rock% snfsdefrag -r -G 2 -k slow -m 0 abc

       Traverse the directory abc list any files that have the affinity fast and having at least one  extent
       in storage pool 2.

          rock% snfsdefrag -r -G 2 -k fast -l -m 0 abc

NOTES
       Only  the  owner  of  a file or superuser is allowed to defragment a file.  (To act as superuser on a
       Xsan volume, in addition to becoming the user root, the configuration option GlobalSuperUser must  be
       enabled.  See snfs_config(5) for more information.)

       snfsdefrag  will  not operate on open files or files that been modified in the past 10 seconds.  If a
       file is modified while defragmentation is in progress, snfsdefrag will abort and  the  file  will  be
       skipped.

       snfsdefrag skips special files and files containing holes.

       snfsdefrag does not follow symbolic links.

       When  operating on a file marked for PerfectFit allocations, snfsdefrag will "do the right thing" and
       preserve the PerfectFit attribute.

       While performing defragmentation, snfsdefrag creates a temporary  file  named  TargetFile__defragtmp.
       If  the  command is interrupted, snfsdefrag will attempt to remove this file.  However, if snfsdefrag
       is killed or a power failure occurs, the temporary file may be left behind.  If snfsdefrag is  subse-quently subsequently
       quently  re-run and attempts defragmentation, it will clean up any stale temporary files encountered.
       But if snfsdefrag is not run again, it will be necessary to find and remove the temporary file as  it
       will  continue to consume space.  Note that user files having the __defragtmp extension should not be
       created if snfsdefrag is to be run.

       snfsdefrag will fail if it cannot locate a set of extents that would reduce the current extent  count
       on a file.

ADVANCED FRAGMENTATION ANALYSIS
       There  are two major types of fragmentation to note: file fragmentation and free space fragmentation.
       File fragmentation is measured by the number of file extents used to store a file. A file extent is a
       contiguous  allocation  unit  within  a file. When a large enough contiguous space cannot be found to
       allocate to a file, multiple smaller  file extents are created. Each extent  represents  a  different
       physical  spot in a storage pool. Requiring multiple extents to address file data impacts performance
       in a number of ways. First, the file system must do more work looking up locations for a file's data.
       In addition, for every ten (10) extents used to address a file's data, a new file inode must be allo-cated allocated
       cated to that file. This will cause increased metadata reads while looking up the locations of  data.
       Also, having file data spread across many different locations in the file system requires the storage
       hardware to do more work while reading a file. On a disk there will be increased head  movements,  as
       the  drive seeks around to read in each data extent.  Many disks also attempt to optimize I/O perfor-mance, performance,
       mance, for example, by attempting to predict upcoming read locations. When a file's data is  contigu-ous contiguous
       ous  these  optimizations  work well. However, with a fragmented file the drive optimizations are not
       nearly as efficient.

       A file's fragmentation should be viewed more as a percentage than as a hard number. While  it's  true
       that  a  file  of  nearly any size with 50000 fragments is extremely fragmented and should be defrag-mented, defragmented,
       mented, a file that has 500 fragments that are mostly one or two FsBlockSize in length is  also  very
       fragmented.  Keeping  files  to  under 10% fragmentation is the ideal, and how close you come to that
       ideal is a compromise based on real-world factors (file system use, file sizes and their  life  span,
       opportunities to run snfsdefrag, etc.).

       In  an attempt to reduce fragmentation (file and free space), Adminstrators can try using the Alloca-tion Allocation
       tion Session Reservation feature.  This feature is managed using the GUI or by modifying  the  Alloc-SessionReservationSize AllocSessionReservationSize
       SessionReservationSize parameter, see snfs_config(5).  See also the Xsan Tuning Guide.

       Some  common  causes of fragmentation are having very full stripe groups (possibly because of affini-ties), affinities),
       ties), a file system that has a lot of fragmented free space (deleting  a  fragmented  file  produces
       fragmented  free  space), heavy use of CIFS or NFS which typically use out of order and cause unopti-mized unoptimized
       mized (uncoalesced) allocations, or an application that writes files in a random order.

       snfsdefrag is designed to detect files which contain file fragmentation and coalesce that data onto a
       minimal  number  of file extents. The efficiency of snfsdefrag is dependent on the  state of the file
       system's free data blocks, or free space.

       The second type of fragmentation is free space fragmentation. The file system's  free  space  is  the
       pool of unallocated data blocks. Space allocation for new files, as well as allocations for extending
       existing files, comes from the file system's free space. Free space fragmentation is measured by  the
       number of fragments of contiguous free blocks.  Fragmentation in the file system's free space affects
       the file system's ability to allocate large extents. A file can only be allocated an extent as  large
       as  the  largest contiguous block of free space. Thus free space fragmentation can lead to file frag-mentation fragmentation
       mentation in larger files. As snfsdefrag processes fragmented files it attempts to use  large  enough
       free space fragments to create a new defragmented file space. If free space is too fragmented snfsde-frag snfsdefrag
       frag may not be able to allocate a large enough extent for the file's data. In the case that  snfsde-frag snfsdefrag
       frag  must  use multiple extents in the defragmented file, it will only proceed if the processed file
       will have less extents than the original.  Otherwise snfsdefrag will abort that file's defrag process
       and move on to remaining defrag requests.

FRAGMENTATION ANALYSIS EXAMPLES
       The  following  examples  include  reporting from snfsdefrag as well as cvfsck. Some examples require
       additional tools such as awk and sort.

       Reporting a specific file's fragmentation (extent count).

              # snfsdefrag -c <filename>

       Report all files, their extents, the total # of files and extents, and the average number of  extents
       per  files.  Beware  that  this command walks the entire file system so it can take a while and cause
       applications to while running.

              # snfsdefrag -r -c -t <mount point>

       The following command will create a report showing each file's path, followed by extent  count,  with
       the  report sorted by extent count. Files with the greatest number of extents will show up at the top
       of the list.

       Replace <fsname> in the following example with the name of your Xsan file system. The report is writ-ten written
       ten to stdout and should be redirected to a file.

              # cvfsck -x <fsname> | awk -F, '{print$6", "$7}' | sort -uk1 -t, \
                | sort -nrk2 -t,

       This  next  command  will display all files with at least 10 extents and with a size of at least 1MB.
       Replace <fsname> in the following example with the name of your Xsan file system. The report is writ-ten written
       ten to stdout and can be redirected to a file.

              # echo "#extents        file size  av. extent size  filename" ;\
                cvfsck -r <fsname> | awk '{if ($3+0 > 1048576 && $5+0 > 10)\
                { printf("%8d %16d %16d  %s\n", $5, $3, $3/$5, $8); }}' | sort -nr

       The  next  command displays a report of free space fragmentation. This allows an administrator to see
       if free space fragmentation may affect future allocation fragmentation. See cvfsck(1)  man  page  for
       description of report output.

              # cvfsck -a -t <fsname>

       The  Fragmentation  detected  RAS warning message may sometimes refer to an inode number instead of a
       file name.  To find the file name associated with the inode number on non-Windows clients,  fill  the
       file  system  mount  point and the decimal inum from the RAS message into the following find command.
       The file name can then be used to defragment the file.  There may be more than one file that  matches
       the 32-bit inode number.

              # find <mount_point> inum <decimal_inum>

              # snfsdefrag <filename>

       For Windows clients:

       Using a DOS shell, CD to the directory containing the StorNext binaries and run the cvstat command as
       follows:  The <fname> parameter is the drive letter:/mount point and the <inum> parameter has  either
       the decimal or hexidecimal 64-bit inode number.  For example:

              c:\> cd c:\Program Files\StorNext\bin

              c:\> cvstat fname=j:\ inum=0x1c0000004183da


FILES
       /Library/Preferences/Xsan/*.cfg

SEE ALSO
       cvfsck(1), cvcp(1), cvmkfile(1), snfs_config(5), cvaffinity(1)



Xsan File System                                December 2011                                  SNFSDEFRAG(1)