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@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
