Re: raw vs block speed difference?

["Michael L. Hitch" <mhitch%lightning.msu.montana.edu@localhost>]

On Thu, 26 Aug 2010, Aaron J. Grier wrote:

> # newfs -b 64k sd3d
> /dev/rsd3d: 17501.8MB (35843664 sectors) block size 65536, fragment size 8192
>        using 6 cylinder groups of 2917.00MB, 46672 blks, 93184 inodes.

  I'm not sure if newfs updates the file system block sizes in the disk 
label anymore, so the label information may not reflect the actual sizes.
In addition, I think the size used is actually the fragment size, which 
is 8K in this case.

> that amount of overhead seems excessive.  am I missing something?

  You can run iostat during the transfer and see what the actual read size 
of the disk is.

  Also, I think using the block device involves a copy of data from the 
kernel to the user process.  The raw device maps the user buffer into a 
kernel address and will transfer the data directly to/from the user buffer 
with no extra copy.

--
Michael L. Hitch                        mhitch%montana.edu@localhost
Computer Consultant
Information Technology Center
Montana State University        Bozeman, MT     USA