Well the original argument  was that if you needed to use a disk at any 
time, the O/S often had a 2GB file size limit.  That mattered if you 
wanted to do D2D backup, or in the rare case of needing to recover data 
from a tape by copying the tape blocks to disk.  That is less of a problem 
as most O/S can support file sizes > 2GB (but do check!).

An argument for large fragment sizes are that the tape record marks 
between the fragments consume space on the tape.  Bigger fragments = fewer 
tape marks = more user data on the tape.  I guess the data flow may slow 
briefly when it is writing the tape marks.  I believe it is not hard to 
work out how much capacity you lose. 

However a significant counter-argument, which many sites will find 
persuasive, is that the restore speed is also affected by the fragment 
size.  In the extreme case, that you don't have a fragment size set at 
all, there is just one fragment on a tape.  Any restore has to scan (at 
slow speed) the whole tape, reading it until the files required for 
restore are found.   With fragments, the tape can skip forward at high 
speed to the required fragment (it is just a record to the tape); it then 
has to read just that fragment at slow speed to get the files.  So unless 
you files are themselves >2GB, you would likely be happy with the 2GB 
fragment size.  It really depends on your data, and how often you do 
restores, how quick they have to be...vs some improved write speed.

William D L Brown




"Jim Langston" <jlangston AT northeastmedical DOT org> 
Sent by: veritas-bu-admin AT mailman.eng.auburn DOT edu
07-Mar-2005 14:31
 
To
veritas-bu AT mailman.eng.auburn DOT edu
cc

Subject
[Veritas-bu] Fragment Sizes...






Folks,

I just upgraded my master and media servers from Datacenter 4.5MP5 to
Enterprise Server 5.0MP4.  I've been running Veritas since 10/03 in my
environment and I've always had a 2 GB fragment size.  My infrastructure
is a StorageTek L700e library with 8 LTO2 drives.  My master server
controls the robot and has 4 drives assigned and my media server has 4
drives assigned. 

I'm getting conflicting recommendations from Veritas support.  Some say
that 2 GB is too small for an LTO2 drive while others say it is fine.
Does anyone else have a similar configuration?  If so, what is your
fragment size?

Thanks,

Jim

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