mkfs.xfs -n size=65536

[Dave Chinner]

On Fri, Oct 09, 2015 at 10:40:00PM +0000, Al Lau (alau2) wrote:
> I am looking for more details on the "-n size=65536" option in
> mkfs.xfs.  The question is the memory allocation this option
> generates.  The system is Redhat EL 7.0
> (3.10.0-229.1.2.el7.x86_64).
> 
> We have been getting this memory allocation deadlock message in
> the /var/log/messages file.  The file system is used for ceph OSD
> and it has about 531894 files.

So, if you only have half a million files being stored, why would you
optimised the directory structure for tens of millions of files in a
single directory?

> Oct  6 07:11:09 abc-ceph1-xyz kernel: XFS: possible memory allocation deadlock in kmem_alloc (mode:0x8250)

mode = ___GFP_WAIT | ___GFP_IO | ___GFP_NOWARN | ___GFP_ZERO
     = GFP_NOFS | GFP_ZERO | GFP_NOWARN

which means it's come through kmem_zalloc() and so is a heap
allocation and hence probably quite small.

Hence I doubt that has anything to do with the directory
block size, as the directory blocks are allocated as single pages
through a completely allocation different path and them virtually
mapped...

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com