[lustre-discuss] Lustre Size Variation after formatiing

[Mohr Jr, Richard Frank (Rick Mohr)]

> On Aug 20, 2018, at 2:58 AM, ANS <ans3456 at gmail.com> wrote:
> 
> 1) CentOS 7.4
> 2) Lustre version 2.11
> 3) MDT LUN Size is 6.5 TB (RAID 10) and after formatting using lustre we are getting the size as 3.9 TB, when formatted using XFS is showing accurate.

For Lustre 2.10 and up, the default inode size is 1KB.  Unless you specifically pass different options to your mkfs command, it will create an inode for every 2.5KB of MDT space.  So the amount of free space will actually be 1.5KB/2.5KB = 60% of the total space.  For your MDT, 0.6 x 6.5TB = 3.9TB. That is the source of the size discrepancy.

--
Rick Mohr
Senior HPC System Administrator
National Institute for Computational Sciences
http://www.nics.tennessee.edu