[Dirvish] Replacing backup drive

Jim Conger jconger at foobox.com
Sat Jul 14 13:34:30 UTC 2018

On 07/14/2018 08:46 AM, Rich Shepard wrote:
> Very interesting. I've not needed to learn about inodes before. Thank you.
Wiki has a good summary of the inode, 
https://en.wikipedia.org/wiki/Inode. Every file is has an inode. The 
inode is the head of the file, it has all the meta data that is not part 
of the file like ownership and dates as well as the link to where the 
file is on disk. Since drivish is collecting files from many source file 
systems into one file system, you may need more inodes than normally 
allocated in the backup file system. Some file systems create inodes on 
the fly, some have a fixed number based on the file system create 
options. You also need an inode for each version of the file over the 
retention period.

This is one of the reasons I use LVM volumes with individual file 
systems for each host I am backing up. I backup five hosts over the 
network and although I can survive all the files from all the file 
systems on one source host in one backup file system, I didn't want to 
put five hosts' worth of files into one file system. I've learned to 
always keep some disk space in reserve so if a host's backup file system 
is getting full, I can expand it. I also use LVM to join smaller drives 
together and mirror across multiple drives.

What dirvish truly beats the snot out of is the directory storage. Say 
you are keeping 30 days of snapshots. If a file hasn't changed in those 
30 days, it is stored once using one inode -- but it is in 30 
directories. If you are investigating file systems, which is a bit like 
trying out religions, don't pick one with limited directory storage or 
poor directory performance.


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