[Lustre-discuss] Size of MDT, used space

[Cliff White]

Thomas Roth wrote:
> 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

The size of the MDS inode depends on the number of stripes. 4.5k is the 
maximum, 512k the minimum. Actually size varies with number of stripes 
in the file. So, we advise using 4k as an estimate, as that will cover 
the vast majority of cases, but actual use in almost all situations will 
be smaller than 4k.
cliffw

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