[Lustre-discuss] Metadata inodes considerations

[Dilger, Andreas]

On 2013-07-15, at 6:20, "Nikolay Kvetsinski" <nkvecinski at gmail.com> wrote:

> Hello, I have a  question about metadata inodes considerations, in the manual says that Lustre metadata is typically 1-2 % of the file system capacity depending upon file size. After that there are some calculations  for average file size of 5 MB and file system size of 100 TB. And the total space for inodes is 80 GB. Do I get it right : 2 % of 100 TB is 2 TB for metadata target. From theese 2 TB, 80 GB will be used for inodes ??  And if I have 1 PB file system,  20 TB for MTD, 800 GB of which for inodes ?

The 1-2% number is just a very conservative estimate if you have no idea what kind of data is being stored. 

If you know the average file size based on existing data that is much more useful. 

1 PB = 1000M MB / 5MB/file = 200M files

200M inodes * 2048 bytes/inode = 400 GB for MDT

Since this is the minimum MDT size for 5MB files, it is recommended to double this to 800GB in case your average file size is shrinking in the future. These days, it is quite reasonable to always use a handful of SSDs for the MDT instead of eg. a single SATA or SAS disk that would have enough capacity. 

Cheers, Andreas