Re: xfs over pmem - cp performance

[Dave Chinner <david@fromorbit.com>]

On Fri, Jan 08, 2016 at 09:07:27PM +0000, Elliott, Robert (Persistent Memory) wrote:
> I tried using cp to copy the linux git tree between
> pmem devices like this:
>                 cp -r /mnt/xfs-pmem1/linux /mnt/xfs-pmem2
> 
> The time taken by various filesystems varies (4.4-rc5):
> * xfs    w/dax: 42 s
> * xfs   no dax: 14 s
> * ext4   w/dax:  7 s
> * ext4  no dax: 15 s
> * btrfs no dax: 18 s

Yes, we know.

> mount options:
> * /dev/pmem1 on /mnt/xfs-pmem1 type xfs (rw,relatime,seclabel,attr2,dax,inode64,noquota)
> * /dev/pmem1 on /mnt/ext4-pmem1 type ext4 (rw,relatime,seclabel,dax,data=ordered)
> * /dev/pmem1 on /mnt/btrfs-pmem1 type btrfs (rw,relatime,seclabel,ssd,space_cache,subvolid=5,subvol=/)
> 
> xfs with dax spends most of the time in clear_page_c_e and
> dax-clear_blocks (from "perf top"): 
>   30.06%  [kernel]            [k] clear_page_c_e        
>   12.24%  [kernel]            [k] dax_clear_blocks      

That's where the difference is - XFS is zeroing the blocks during
allocation so that we know that a failed write or crash during a
write will not expose stale data to the user. I've made comment
about this previously here:

http://oss.sgi.com/archives/xfs/2015-11/msg00021.html

and it's a result of the current "everything is synchronous" DAX cpu
cache control behaviour.

I think it's worth noting that ext4 is not spending any time
zeroing the blocks during allocation, which I think means that it
can expose stale data as a result of a crash or partial write....

We're working on fixing this, but it needs all the fsync patches
from Ross to enable us to turn off the synchronous cache flushes
in the DAX IO code.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com