ssd time to make head positioning (latency): <0.1ms
hd max time to make head positioning (latency): 10ms

ssd rate of read: 270MB/s random/sequential read (excluding latency)
check that ssd is BLOCK (4kb mostly) oriented
hd rate of read? 130MB/s sequential read? check that hd is BIT oriented
write rate? random/sequencial?

with these answers we can make a simple 'time' model of read/write,
per device (use of raid0 (/dev/md0) is a device!, raid1 too
(/dev/md1), raid5 (/dev/md2) ,raid6 (/dev/md3)) any device have this
variables...
just make a model and use the model to optimize minimal time to
execute write/read

2011/2/2 Roberto Spadim [off-list ref]:

pros against closest head:
since we can use raid1 with identical disks (buyed at same time, with
near serial numbers) we can have disks with same time to fail
using closest head, the more used disk, will fail first
failing first we have time to change it (while the second isn't as
used as first device)

but, think about it...
it's like a write-mostly not?


2011/2/2 Roberto Spadim [off-list ref]:

quoted

check that, read balance is:
time based
closest head
round robin
algorithms

plus....
failed device problem and write-mostly

with time based we can drop write-mosty.... just make the time of that
device very high


2011/2/2 Roberto Spadim [off-list ref]:

quoted

it's cpu/mem consuming if use a complex model, and less cpu/mem
consuming if use a single model

another idea....
many algorithm....

first execute time based
it selected a bug (failed) device
execute closest head
if selected a bug (failed) device
execute round robin
if selected a bug (failed) device
select first usable non write-mostly
if selected a bug (failed) device
select first usable write-mostly
if end of devices, stop md raid

to make this, today... we need a read_algorithm at
/sys/block/md0/xxxxxx, to select what algorith to use, write algorithm
is based on raid being used.. raid0 make linear and stripe, raid1 make
mirror, there's no algorithm to use here...
we need some files at /sys/block/md0/xxx to manage 'devices' time
model (parameters)
we need a adaptive algorithm to update parameters and make it closest
possible to real model of 'devices'
a raid0 have global parameters, inside raid0 devices have per device parameters
a raid1 over raid0, should use raid0 parameters
raid0 over devices, should use devices parameters


2011/2/2 Roberto Spadim [off-list ref]:

quoted

time based: is the time to:
HD:head positioning , SSD: time to send command to ROM chip
HD:read/write time (disk speed - rpm), SSD: time to write/read (time
to ssd rom chip receive bytes)
that's time based

what is fast por read?
consider that time based must know that disk is doing a I/O and that
you have a time to end, this time to end is another time in algorithm

for example:
NBD (network block device)
time to send read message + time to send command to rom or head positioning
read/write time: time to nbd server return the read/write bytes

what algorithm should do?
calculate all time or all mirrors, including time to end current
request (if only one request could be processed, or if allow more than
1 request, the time spent to start our command)
after all time calculated, select the minimal value/device

that's time based
it's not based on round robin
it's not based on closest head
it's based on device speed to:
*(1)position head/send rom command
*(2)read/write time (per total of bytes read/write)
*(3)time to start out request command (if don't allow more than 1
request per time, don't have a device queue)

the total time per device will tell us the best device to read
if we mix, nbd + ssd + hdd (5000rpm) + hdd(7500rpm) + hdd(10000rpm) +
hdd(15000rpm)
we can get the best read time using this algorithm
the problem? we must run a constante benchmark to get this values *(1)
*(2) *(3) and calculate good values of time spent on each process

resuming... whe need a model of each device (simple-constants or very
complex-neural network?), and calculate time spent per device
nice?


2011/2/2 Robin Hill [off-list ref]:

quoted

On Tue Feb 01, 2011 at 09:12:11PM -0200, Roberto Spadim wrote:

quoted

but the best algorithm is time based (minimize time to access data)

And what do you think takes the time accessing the data?  In a rotating
disk, it's moving the heads - that's why the current strategy is nearest
head.  In an SSD there's no head movement, so access time should be the
same for accessing any data, making it pretty much irrelevant which
strategy is used.

Cheers,
   Robin

--
Roberto Spadim
Spadim Technology / SPAEmpresarial

--
Roberto Spadim
Spadim Technology / SPAEmpresarial

--
Roberto Spadim
Spadim Technology / SPAEmpresarial

--
Roberto Spadim
Spadim Technology / SPAEmpresarial

-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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