hummm well that´s true... exist a queue inside disk hardware that we
can´t measure... but... if you want i can make tests to you :)
i used a configuration a bit diferent some time ago, instead of a SSD
and a harddisk, i used a disk with 7200rpm and a disk with 15000 the
"time based" algorithm runs nice in this case, maybe could give just a
little more 'performace' (maybe none), like i told the mean performace
that i got was 1% (i made tests with different disks speed and
ssd+disks, i had a ocz vortex2, a sata 7200rpm (500gb) and a sas
15000rpm (142gb), some other guy here in kernel list tested too, but
they didn´t confirmed if the performace was a mean performace or just
a 'error' in measure
when i done this i got some 'empirical' values to 'tune' the
algorithm, i don´t remember all 'theory' but i done something like
this:
1) (distance * time/distance unit)
time/distance unit,
i don´t remember distance unit, i think it´s 1 block = 512bytes
right? well, just check the idea...
for disks:
total disk capacity in distance units / 1 revolution time
1 revolution time = 1/rpm for disk, for example
7200 rpm => 120 hz => 8.333ms = 8333us (near 10ms like
told in disk spec of random acess time)
15000 rpm => 250hz => 4ms = 4000us (near 5ms like told
in disk spec)
for ssd : 0 seconds
7200 => 500gb (1024*1024*1024/512) / 8333 = 1048576000blocks /
8333us = 0.000'007'946'968'078 block/us
15000 => 142gb (1024*1024*1024/512) / 4000us = 297795584blocks /
4000us = 0.000'013'432'032'625 block/us
ssd => infinite blocks/us
0.000007946 for 7200rpm,
0.000013432 for 15000rpm,
0 for ssd
2)(blocks to read/write * time to read/write 1 block)
this part i put dd to work...
dd if=/dev/sda of=/dev/null (there was some flags to remove cache
too but don´t remember now...)
and used iostat -d 1 -k to get mean read performace
i don´t remember the rights numbers but they was something near this:
ssd - 230mb/s = 230Mb(1024*1024)/512bytes => 471040 blocks /
second = 0.000'002'122 => 2.122us / block
hd 7200 - 120mb/s => 245760 blocks/second => 0.000'004'069 =>
4.069us / block
hd 15000 - 170mb/s => 348160 blocks/second => 0.000'002'872 =>
2.872us / block
3) (non sequencial penalty time)
here i used two dd to do this (some seconds between first and second dd)
and got the new mb/s values
ssd get a bit down but not much 230 -> 200
hd 7200 120mb -> 90
hd 15000 170 -> 150
with this loses i done a 'penalty' value
(230-200)/230 = 13.043%
(120-90)/120 = 25%
(170-150)/170 = 11.76%
i don´t remember if i used the penalty with distance=0, or if i used
it like in today implementation that select the previous disk when
reading the full md device
======
with this numbers.... some algorithms expected selects...
sda=ssd, sdb=15000rpm, sdc=7200rpm
sda|sdb|sdc
disk positions: 0 | 0 | 0
read 100 block at position 20000...
sda=> distance = 20000, extimate time = 20000*0 + 2.122*100 + 13.043%
in other words...
( 0 + 212.2) * 1.13043 = 239.877246
sdb=> distance = 20000, extimate time = 20000*0.000013432 + 2.872*100
+ 11.76% =
(0.26864 + 287.2) * 1.1176 = 321.274952064
sdc=> distance = 20000, extimate time = 20000*0.000007946 + 4.069*100 + 25% =
(0.15892 + 406.9) * 1.25 = 508.82365
HERE WE SELECT sda (239.877)
disk positions: 200 | 0 | 0
read 100 blocks at position 0...
sda=> distance = 200, extimate time = 200*0 + 2.122*100 + 13.043%
( 0 + 212.2) * 1.13043 = 239.877246
sdb=> distance = 0, extimate time = 0*0.000013432 + 2.872*100 + 0% =
(no penalty here since we are at the right place)
( 0 + 287.2) * 1 = 287.2
sdc=> distance = 0, extimate time = 0*0.000007946 + 4.069*100 + 0% =
( 0 + 406.9) * 1 = 406.9
sda...
check that i will always select sda... since it´s fast for distance
(0seconds) and have the highets transfer rate
that´s here my algorithm didn´t worked fine... (i don´t know anything
about past and queue just the current read)
but now... with someone that know the kernel code... we have this
information of pendings requests =D
i think we can go inside queue and calculate the total estimate time =), or not?
for each pending request we should calculate this times... and sum
the total time to select the 'best' disk
here i didn´t coded since i don´t know how to get information from
queue in kernel =( and my hobby ended ='(
thanks to read....
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