Heh, would help if I actually attached the file ;)


Phillip Susi wrote:
> Attached are the results of some simple testing I did in ods format. 
> These tests involved having dd read the first GB of data from my two 
> drive sata (fake)raid0 array with varying numbers of concurrent aio 
> operations ( except for the original, non aio dd of course ).
> 
> I performed these tests with cpufreq disabled and filesystems mounted 
> with noatime to insure no disturbances.  I also set the IO scheduler to 
> noop, otherwise the default scheduler reordered the IO requests which 
> was not good for sequential throughput.  I used commands like this:
> 
> sync
> dd bs=512 count=1 iflag=direct if=/dev/sda of=/dev/null
> dd bs=512 count=1 iflag=direct if=/dev/sdb of=/dev/null
> time dd bs=128KiB count=32768 iflag=direct if=/dev/mapper/via_hfciifae 
> of=/dev/null
> 
> The first two commands were to make sure the drive head was on track 
> zero, otherwise the TCQ on the drives kicked in and reordered some of 
> the earlier reads as the head seeked to track zero.
> 
> The results show a rather large increase in throughput for block sizes 
> under 128 KB, with a smaller improvement on larger block size. Likewise, 
> the cpu time used was significantly lower, especially with block sizes 
> less than 128 KB.  In most cases, the original dd uses 2-3 times more 
> cpu time than the aio dd.
> 
> The original dd reached near peak throughput ( 93.4 MB/s ) at a block 
> size of 128 KB.  I believe this is due in part to that being the stripe 
> width of the array, so smaller block sizes did not keep both drives 
> operating full time.  In contrast all of the aio trials reached peak 
> throughput of 97.x MB/s with a block size of only 32 KB, and at the 
> smallest block size of 16 KB, the aio(16) trial managed more than 20% 
> higher throughput than the non aio dd ( 72.1 vs  59.7 MB/s ), and did so 
> using 1/7th the cpu time.
> 
> To show the difference O_DIRECT makes, at 128 KB block size the original 
> dd with O_DIRECT managed 93.4 MB/s using 0.906 seconds of CPU time. 
> Without O_DIRECT, the original dd only sustains 82.6 MB/s and uses a 
> whopping 2.912 seconds of cpu time, or more than triple the time without 
> O_DIRECT, and 13x more cpu time than the aio(4) test at that block size!
>