Network Support Library/RoseWare - Network Support Library.iso/pressgen/frag.txt

Text File | 1988-09-17 | 10KB | 173 lines

From APRIL 1988 LAN TIMES, page 28.

Fragments In Performance

By John T. McCann

     Everyone  wants  more  performance from a  network.  For  personal
computer (PC) users,  defragmenting their hard disk might be a suitable
remedy.  But,  if the same PC users, who have just turned LAN managers,
attempt  to  defragment their network's shared disk,  they'll  find  it
difficult or impossible to locate such a utility for a NetWare network.

WHAT'S A FRAGMENTED FILE?
     A  file is called fragmented when it does not  exist  contiguously
(in  a sequential order) on a disk.  Accessing a fragmented file  means
much  time can be wasted waiting for the drive to position  (move)  the
disk's  heads  to  the  next  portion  of  the  file.      Higher  disk
performance is gained from disks that spend less time positioning their
heads,   resulting  in  a  quicker  response  time  and  better  system
performance.
     On  a  standalone  PC,   higher  disk  performance  is  based   on
contiguously allocated files. Activity of the disk's heads is minimized
to  process  (read  or write) such  files.  Consequently,  contiguously
allocated  files  in a single-user system allow for the  quickest  file
access.

THE MULTIUSER DISK
     In a multiuser environment,  contiguous files are most  beneficial
with single-user access.  But unlike on a standalone,  fragmented files
on a shared hard disk are generally not a problem.  To explain why this
is not a problem, some common background needs to be established.

REASONS FOR ACCESSING FILES
     There  are  two basic reasons to access files:  for  their  entire
content or for certain segments within the file.
     People  who  access files for their entire content  are  generally
calling up programs, whether compilers, text editors,  word processors,
utilities,   or  applications.   Some  programs  also   need   overlay,
configuration, help, or driver files. Typically, these additional files
are used when a program initially comes up.  Word processing  documents
and spreadsheet data are additional examples of files that are accessed
for their entire content.
     Users  who need only file segments might be calling  up  database,
index  and non-database data files. File segments are  accessed  either
sequentially or randomly.  Users call up file segments much more  often
than they call up initial programs and their additional files.
     A multiuser system can experience fragmentation degradation if the
system  is using First-In-First-Out (FIFO) queue mechanisms  to  access
files.  FIFO mechanisms are DOS-like by nature and perform each read or
write request in order of arrival at the file server, no matter how far
removed the areas to be accessed are from each other.
     FIFO  access  in  a multiuser system can also lead  to  hard  disk
thrashing.  A  disk that spends more time positioning  its  heads  than
accessing data is said to be thrashing.

THE NETWARE SOLUTION
     NetWare minimizes thrashing by using an elevator-seeking mechanism
that  logically organizes disk operations as they arrive at the  server
for processing.  As a consequence of elevator-seeking,  the drive heads
operate in a sweeping fashion, from one edge of the disk to the other.
     This intervention by NetWare ensures high throughput from the disk
regardless   of  the  amount  of  activity  requested  or  the   actual
fragmentation of the files being accessed.
     In  addition to elevator-seeking,  NetWare also employs a  caching
mechanism. Caching stores in the server's memory the most recently read
disk  blocks (one disk block is 4KB).  Caching also  temporarily  holds
blocks to be written to disk.
     If the requested data exists in cache,  no physical disk  activity
will  occur to retrieve the data. Writing data to the  cache  minimizes
the  wait  time for the user.  Instead of waiting for the  data  to  be
written  to  the  disk,  the data is written to  cache  and  users  can
continue  their operations.  Cache-stored data will be written to  disk
every three seconds, or when the entire cache buffer is updated.
     For example,  if a file is being updated,  NetWare would store the
data  in  the cache.  Once the data is in the  cache,  the  application
program continues because it thinks the data has been written to  disk.
And  if data in the cache is needed for a read operation before  it  is
written to disk, it will be read from the cache.
     NetWare  speeds  performance  through elevator  seeking  and  disk
caching.  The goal of elevator seeking is to reduce the amount of  time
spent positioning a disk's heads,  while caching eliminates unnecessary
disk  access.  Together,  elevator seeking  and  caching  diminish  the
effects of fragmentation in the NetWare environment.

V2.1X FEATURES FAT INDEXING
     Elevator  seeking and caching features are representative  of  all
versions  of Advanced NetWare.  But,  with the arrival of  SFT  NetWare
v2.1,  Novell  is introducing a new feature to  further  decrease  file
access time.
     The new feature is File Allocation Table (FAT) indexing.  The  FAT
is  a singly linked list of pointers to disk blocks  containing  files.
Further,  the FAT is the device which resolves the  logical-to-physical
addresses needed to access files. FAT indexing decreases the time spent
finding a specific FAT entry.
     Access to a certain FAT entry requires the sequential scanning  of
all  entries previous to it.  A file marked for FAT indexing (which  is
much like marking a file to be shareable) has a dynamically constructed
index table containing pointers to that file.
     Using the index table,  NetWare jumps instantly to the needed  FAT
entry  (rather  than  scanning  all  previous  FAT  entries)  on  files
containing no more than 512 4KB blocks or totalling no more than 2MB.
     If a file is initially greater than 2MB, or grows beyond 2MB,  the
indexed  FAT  dynamically begins to index every other FAT  entry.  Even
though  the  file's  size is greater than 2MB and  the  number  of  FAT
entries  pointed  to  by each index  entry  is  increased,  performance
remains higher than sequential FAT access because the FAT index reduces
sequential searches to a very small number.
     Up to 1,000 FAT index tables per server are allowed by SFT NetWare
v2.1. Each FAT index table requires 1KB of memory at the server.
     FAT  indexing is best implemented on files that are  accessed  for
specific segments rather than files accessed for their entire  content.
(Files read for their content are read sequentially,  the same way  the
FAT works.)

SOME ALTERNATIVES
     If fragmented files on a NetWare or other network shared hard disk
is  still a concern,  here are some salient areas to consider that  are
proven to increase network performance:

~  Add memory in the file server (more is better).*
~   Adjust  the cache block size (with versions of  NetWare  v2.0a  and
below,  cache block size can be changed;  4K block size is optimal  for
system speed but uses more memory).*
~   Increase the speed of the disk channel in the server  (i.e.,  maybe
add a DCB for 286 NetWare).*
~  Decrease the average access time of shared disk(s).
~  Increase the processing speed of network workstations.
~  Increase the number of network cards in the file server and split up
the network cabling to decrease
communication congestion between file server and workstations.*

     Performance can increase 10-fold or more by utilizing one or  more
of the above improveme
nts. (The asterisks indicate NetWare-specific enhancements.)
     If none of these enhancements seem adequate,  here is a method  to
defragment any disk:

~   Time certain activities utilizing the disk;  maybe load a  program,
spreadsheet, etc. Try this with no other users logged on the system.
~  Back up the system completely,  at least twice.  (If it's a  NetWare
system,  and  not  using v2.1,  use some  system  other  than  Novell's
internal tape unit to perform the backup and restore.)
~   CompSurf  the  disk  in question.  (If  non-NetWare,  reformat  the
specific disk.)
~  Re-install that disk into the system.
~  Restore the system.
~  Do the same time tests as before.
~  Ask the question, "Was it worth it?"

     The  above  steps were performed on  several  NetWare-shared  hard
disks  and  produced no noticeable increase in performance.  The  disks
were all larger than 100MB and had been in service for more than a year
before the test.
     The  whole  premise of fragmented files  limiting  performance  is
based  on  a  single-user system,  or a shared  system  that  practices
single-user  disk  access techniques (such as FIFO).  The fact  that  a
shared disk is in constant motion, due to multiuser access,  supersedes
the benefit of having files contiguously organized.
     Unless a file is processed for its entire content, the next time a
file segment is processed, repositioning disk heads will most likely be
necessary  because the disk heads will probably have moved  to  service
another  user's  request.  Consequently, fragmented files  have  little
effect on system performance in a shared disk environment.
     Through elevator seeking and caching,  decreased performance of  a
NetWare system cannot be directly attributed to fragmented files.

John T.  McCann is the author of the Integrity Software's ANET  Network
Utilities  and SiteLock and has installed over 500 NetWare  systems  in
the past two and a half years.  He is also the Wizard SysOp on  NOVUSER
NetWire,   and   can  be  reached  at  CompuServe  ID   70007,3430   or
1-800-752-7001 x2807