[Lustre-discuss] Size of MDT, used space

[Thomas Roth]

Hi all,

I'm still in trouble with numbers: the available, used and necessary 
space on my MDT:
According to "lfs df", I have now filled my file system with 115.3 TB.
All of these files are sized 5 MB. That should be roughly 24 million files.
For the MDT, "lfs df" reports 28.2 GB used.

Now I believed that creating a file on Lustre means using one inode on 
the MDT. Since all of my Lustre partitions were formatted with the 
default options (all of this is running Lustre v. 1.6.4.3, btw), an 
inode should eat up 4kB on the MDT partition. Of course, 24 million 
files times 4 kB gives you 91 GB rather than 28GB.
Obviously, there is something I missed completely. Perhaps somebody 
could illuminate me here?

This issue could also be phrased as "How large should my MDT be to 
accommodate n TB storage space?" The manual's answer boils down to "= 
number of files * 4 kB "  (*2 per recommendation). That's how I 
calculated above - maybe my test system is broken? I can't check on the 
content of these files, it's just 5MB test files created with the 
'stress' utility.

  Thanks and regards,
Thomas