
LIMITATIONS :
==============
The size of SLICES may be limited by the file system or kernel (maximum
file size is 2 GB with Linux kernel 2.2.x), the number of SLICES is
only limited by the size of the filenames, thus using a basename of 10
chars, considering your file system can support 256 char per filename at
most, you could already get up to  10^241 SLICES, 1 followed by 241 zero.
But as soon as your file system will support bigger files or longer filename,
dar will follow without change.

dar_manager can gather up to 65534 different backups. This limit should be
high enough to not be a problem.

Memory Issue
-------------
dar uses virtual memory (= RAM+swap) to be able to add the list of file saved
at the end of each archive. Dar uses its own integer type (called "infinint")
that do not have limit (unlike 32 bits or 64 bits integers). This makes dar
already able to manage Zettabytes volumes and above even if the systems cannot
yet manage such file sizes. Nevertheless, this has an overhead with memory and
CPU usage, added to the C++ overhead for the data structure. All together dar
needs a average of 650 bytes of virtual memory by saved file. Thus, for example
if you have 110,000 files to save, whatever is the total amount of data to
save, dar will require around 67 MB of virtual memory.

Now, when doing catalogue extraction or differential backup, dar has in memory
two catalogues, thus the amount of memory space needed is the double (134 MB
in the example). Why ? Because for differential backup, dar starts with the
catalogue of the archive of reference which is needed to know which files to
save and which not to save, and in another hand, builds the catalogue of the
new archive all along the process. Now, for catalogue extraction, the process
is equivalent to making a differential backup just after a full backup.

This memory issue, is not a limit by itself, but you need enough virtual
memory to be able to save your data (if necessary you can still add swap
space).

Special integer mode
---------------------
To overcome the previously explained memory issue, dar can be build in an
other mode. In this other mode "infinint" is replaced by 32 bits or 64 bits
integers, as defined by the use of --enable-mode=32 or --enable-mode=64
options given to configure script. The executables built this way (dar,
dar_xform, dar_slave and dar_manager) run faster and use much less memory than
the "full" versions using "infinint". But yes, there are drawbacks:

slice size, file size, dates, number of files to backup, total archive size
(sum of all slices), etc, are bounded by the maximum value of the used integer,
which is 4,294,967,296 for 32 bits and 18,446,744,073,709,551,616 for 64 bits
integers.

What the comportment when such a limit is reached ?
For compatibility with the rest of the code limited length integers (32 or 64
for now) cannot be used as-is, they are enclosed in a C++ class, which will
report overflow in arithmetic operations. Archives generated with all the
different version of dar will stay compatible between them, but the 32 bits or
64 bits will not be able to read or produce all possible archive. In that
case, dar suite program will abort with an error message asking you to use the
"full" version of dar program.


Long/short options
-------------------
on several system command-line long options are not available. Dar actually 
relies on the system library. In the future, dar will come with its own 
getopt_long call to overcome this problem. Solaris is one of the affected 
system.



