Boniforti Flavio wrote:
> Hello list,
> 
> I think I haven't yet completely understood what pooling is and how it
> works: does anybody have a link for a good explanation and/or is wanting
> to explain it here?

It is simple if you understand how hard links work in unix filesystes. 
The real information about a file is stored in the inode, while the 
filename in the directory is just a link to this inode, which maintains 
a count of links.  There can be many or no names (links) pointing to the 
same inode, and most directory operations are atomic.  The contents of 
files remain available on the disk until there are no links remaining 
and no process has it open.

Backuppc does a hash over some portion of the file contents and uses 
that as the name in the pool directory (cpool if compression is on but 
the hash is for the uncompressed data). If this name doesn't exist, it 
is created as a link to the inode of the new file in a backup.  If it 
does, there are some more extensive tests for collisions and on a match 
the new backup file is replaced by a link to the pooled copy.  The file 
attributes (owner/modes, etc.) are maintained separately in the attrib 
file in the backup directory since these may be different for different 
instances of the same content.  This approach works whether the content 
is duplicated in multiple backup runs on the same machine or the same 
content is found on many machines.  A side effect of this technique is 
that the pool and pc directories (and everything below) must always be 
on the same filesystem.

-- 
   Les Mikesell
    lesmikesell AT gmail DOT com



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