On 12/06/2014 03:40 PM, Eric Wheeler wrote:
On Fri, 5 Dec 2014, Mike Snitzer wrote:
I do wonder what the performance impact is on this for dm. Have you
tried a (worst case) test of writing blocks that are zero filled, but
with the last byte not being a zero?
The additional overhead of worst-case should be (nearly) equal to the
simplest test case of dd if=/dev/zero of=/dev/thinp/vol. In my testing
that was 1.4GB/s within KVM on an Intel Xeon(R) CPU E3-1230 V2 @ 3.30GHz.
That seems way too slow for checking if it's zero or not... Memory
bandwidth should be way higher than that. The line above, was that what
you ran? How does it look with bs=4k or higher?
I could see where really fast storage that can obtain 1.4gb/s might
notice a performance regression---but for most use cases that is quite
fast.
Depends on your view point, I think it's pretty slow. Plenty of devices
out there that are faster than that. Quick example, the laptop I am
typing this email from:
read : io=12233MB, bw=1432.7MB/s, iops=22922, runt= 8539msec
Writes are around 1GB/sec, enough that I bet the slowdown in checking
for zeroes will be noticeable. In the pci-e flash space, more than
double your current /dev/zero benchmark is common for current devices.
--
Jens Axboe
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