On 08/03/2010 05:57 PM, John Leach wrote:
On Tue, 2010-08-03 at 17:44 +0300, Avi Kivity wrote:
On 08/03/2010 05:40 PM, John Leach wrote:
dd if=/dev/mapper/zero of=/dev/null bs=8k count=1000000 iflag=direct
8192000000 bytes (8.2 GB) copied, 3.46529 s, 2.4 GB/s
dd if=/dev/mapper/zero of=/dev/null bs=8k count=1000000
8192000000 bytes (8.2 GB) copied, 5.5741 s, 1.5 GB/s
dd is just using read.
What's /dev/mapper/zero? A real volume or a zero target?
zero target:
echo "0 21474836480 zero" | dmsetup create zero
The same performance penalty occurs when using real disks though, I just
moved to a zero target to rule out the variables of spinning metal and
raid controller caches.
Don't, it's confusing things. I'd expect dd to be slower with
iflag=direct since the kernel can't do readahead an instead must
roundtrip to the controller. With a zero target it's faster since it
doesn't have to roundtrip and instead avoids a copy.
--
error compiling committee.c: too many arguments to function
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