Re: [amibroker] Re: Building a 1-minute historical Database

[Herman <psytek@xxxxxxxx>]

Thanks Tim, regretfully it is not clear to me.

for example DTN is a realtime data plugin but, afaik, does not keep its own separate DB. Does changing the local setting then wipe out RT data? I use DTN and the number of bars in my DB don't seem to be effected by any settings.
I use my DTN RT database in local mode, for speedy AA operation, but connect it for backfill. Since we have separate local and RT settings. How does that work?

Herman


tr4d3rt wrote:

Herman,

In regards to question #1, I emailed support a week or two ago with the same question (as I noticed that it had no effect in limiting data in a local db). This is how Marcin explained it:

"This setting applies to the databases operated by RT plugin, but not to
(local database). If you want to limit the size of the local database - then
go to Tools -> Preferences -> Data, then mark "Limit number of saved
qutations" (and modify the data somehow, so the changes will be saved and
limit applied with the save operation)."


- Tim

On Thu, Oct 8, 2009 at 9:54 AM, Herman <psytek@xxxxxxxx> wrote:

For DB backups I use dated sub-folders in an AmiBrokerBackup folder. I
don't overwrite anything, I just delete a bunch once in awhile. Disk
space is plentiful and cheap but time is not. When backing up and
restoring AB be aware that you can run into problems restoring if you
backup to a different drive letter.

As someone else mentioned, for processing speed I develop with my large
RT DB set to "Local database", having the data plugin connected really
slows down things.

There are other factors to consider when working with minute DBs. A
clear understanding of what the DB and Preference settings mean, and how
they should be set is essential. After all these years i am still
struggling with some points, perhaps someone can explain in layman's terms:

1) What does "File -> Database Settings -> Number of bars" really mean?
It doesn't seem to set the "limit" for the data base because I can still
have 250000 bars even if it is set to 50,000. If it refers to "Bars
Loaded" why do we have another "Number of Bars to load" in Preferences
-> Data ? When is it enforced, only when the DB is created or can it be
changed for an old DB? Would this corrupt my DB? Would it reduce the DB
in Size?

2) Is there an absolute limit to the number of bars loaded? What happens
when you exceed the DB or Preferences setting, would it corrupt the DB?

3) How to set the In-Memory Cache size? Do we have to change this each
time we run a different DB? Perhaps someone should design a AmiBroker
Setup calculator :-) one of those panels you enter your system/DB
parameters in and it tells you what settings to use. Volunteers?

4) Same question for Max. megabytes...

When changing these settings, when do they take effect?

I know there are ways to calculate memory usage but I create a new DB
almost every day, I wish there was a safe default setting (XP 4GB). If
CacheSize * NumberOfBars * 32 cannot exceed the maxMegabytes the user
should be prevented from doing so. Or is it more complicated that that?

Often when running a portfolio Backtest everything runs fine until at
the very end (after waiting ten minutes) the Backtester gives me an
error. I am pretty sure i am running out of memory, but this is a
technical issue that i shouldn't have to concern myself with.

herman

etoketrader wrote:
> touching on the topic of backups, I fully agree with Herman.
> In my nightly schedule I shut down AB and copy the entire folder as follows:
>
> AB3 -> AB4
> AB2 -> AB3
> AB1 -> AB2
> Live Amimbroker -> AB1
>
> Sounds like an overkill, but it is not the first time I had to go back several copies. Corrupt data is not immediately evident in many cases, especially when dealing with large DB's.
>
> BR,
> eToke
>
>
>
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