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3.1.1 Epoch vs Unitree


This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.

3.1.1 Epoch vs Unitree

    	(Note: this evaluation is old, and should be taken with a
grain of salt. rdv, 3/96)

        (6/93) We just bought both last year.  We bought an Epoch I
with the 20 GB EO and 327 GB worm.  We will be upgrading it to an
Epoch II soon.  We also bought Unitree from Titan to run on a Silicon
Graphics server and hook up to the STK 3480 silo.  We hope to add more
silos eventually.

Unitree is licensed based on storage capacity while Epoch is not.
There may be an exception to this - STK just began reselling Epoch as
the front end for their silos and I'm not sure how they handle

My office mate and I (I handle Epoch, he handles Unitree) have enjoyed
comparing the merits/demerits of each over the last year.  Comparison
in our case is slightly slanted due to the fact that the Epoch has
optical disk while the Unitree system has 3480 tape - so some
observations have more to do with media advantages/disadvantages.


+  Allows large files - can span volumes
+  Allows you to enlarge the cache easily, allows very large 
+- Unitree has replaced several UNIX utilities with their own 
(FTP, NFS and the file system).  This allows certain features to 
work but is generally slower and disallows access to the archive when 
you are on the server itself - unless you NFS mount!
+  Allows deleted files to be saved for a specified time
+  Allows multiple copies of files to be kept
+  Data is copied to archive soon after creation
+  Unitree runs on several different platforms
-  Does not allow access to data until it is completely 
-  Behaves poorly with small files (due to necessary overhead)
-  Unitree is licensed to several vendors, so versions differ
-  NFS access is so slow that we recommend it not be used for 
file transfer - only for ls and du, etc.  Use FTP.
-  The Silicon Graphics version is still new and has some 


+  Allows access to the data as soon as part of it is loaded
+  Company seems serious about reputation and support
+  The Epoch II is based on a SUN system, with few 
+  Data is copied to archive only when the cache space is 
+  All native UNIX transfer methods work - NFS, FTP and RCP
+  Add on products greatly simplify backup and extend 
archiving features to other systems.
-  Deleted files are gone forever
-  Currently only available on SUN.  This will change.
-  Cannot span volumes yet - limiting file size
-  Has the SUN limitation of 2 Gb per filesystem.  This would 
be a bigger problem if you used it for a 3480 silo.
{Note 2GB of Magnetic Disk limit, not the entire HSM store}
-  Behaves poorly with small files (due to necessary overhead)
-  Since inodes are kept on magnetic cache, you must take 
into account the maximum number of files you will ever need.
-  Since inodes are always on disk, certain disk operations 
can take forever since all inodes must be examined.
-  Enlarging a magnetic disk filesystem which has associated 
archive media requires you to offload all data and then reload it.  
If anyone has found another way, I would like to hear about 
                   {Others did offer some easier work-arounds}

In all fairness to Titan, they have been addressing any problems and
it has been improving.  Epoch too plans to address some of their
shortcomings.  We are looking forward to growing with both products.

The likelihood that the various flavors of Unitree will standardize
depends on what happens with Discos.  My guess is that some
features/enhancements will be filtered back to the base product
released by Discos.  Bye...

        (bodoh@dgg.cr.usgs.gov,, Tom Bodoh, USGS/EROS Data 
Center, Sioux Falls, SD)


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