[Lustre-discuss] MDS sizing question

[Craig Prescott]

Hi all;

I am considering hardware requirements for an
MDS to be paired with a 500TB Lustre filesystem.
I have a question regarding the sizing guidelines
described in the manual.

For an anticipated average file size of 1MB, the
MDT size guideline from section 21.3.2 works out
to 4TB.  For comparison, on our production 28TB
Lustre filesystem, we have:

# lfs df
UUID                 1K-blocks      Used Available  Use% Mounted on
ufhpc-MDT0000_UUID   213655168  17329964 196325204    8% 
/ufhpc/scratch[MDT:0]
...
filesystem summary:  29966190744 20728270148 9237920596   69% /ufhpc/scratch

# lfs df -i
UUID                    Inodes     IUsed     IFree IUse% Mounted on
ufhpc-MDT0000_UUID    61049728  21416476  39633252   35% 
/ufhpc/scratch[MDT:0]
...
filesystem summary:   61049728  21416476  39633252   35% /ufhpc/scratch

My concern is that if I follow the guidelines, I would
over-provision the MDS with space we would never use.

I understand the inodes are pre-allocated and won't show up in
the "Used" column above.  Under what conditions would the actual
MDT space get used more significantly?

Thanks,
Craig Prescott
UF HPC Center