[Lustre-discuss] question about size on MDS (MDT) for lustre-1.8

[Andreas Dilger]

On 2011-02-01, at 11:38, Jason Rappleye <jason.rappleye at nasa.gov> wrote:
>  I heard through the grapevine that you suggest not using "too few" flex_bgs on an ext4 filesystem. Can you elaborate on what might be a reasonable number, and why?

My gut feeling is that a flex_bg factor of 256 may give the best tradeoff of performance and configurability for a hybrid storage device. That will allow bitmaps and itables to be multiples of 1MB sizes, and with careful tuning they can also be aligned on 1MB boundaries.

It means that 1/256th of the filesystem would be allocated on SSD storage (4GB per TB) which I think is totally reasonable (64GB for a 16TB filesystem), while at the same time avoiding too complex an LV layout (512 SSD regions for a 16TB filesystem).

In the end we need to do the testing to know what the best tradeoff is. 

Cheers, Andreas