Re: [Gumstix-users] Databases and transactions...

[Dan T. <log...@gm...>]

Thanks for the additional thoughts on this...

I think what I'll do to start with is store logs in memory and then
save periodically to MMC, perhaps once a minute.  Since I was only
planning to use the DBMS to manage concurrent access I think I can get
away with using flat output files now.  Keeping things in RAM will
allow me to run length encode data - by only saving new logs when the
position has actually changed.  Using the MMC for storage will allow
me to easily transfer the files in places where I don't have software
to connect on USBnet or serial.

I was worried about writing to MMC too often, but Craig's comments
have cheered me up :o)

Cheers,

Dan

On 14/06/05, David I S Mandala <da...@th...> wrote:
> On Tue, 2005-06-14 at 16:27 -0400, Craig Hughes wrote:
> > On Jun 14, 2005, at 10:31 AM, David I S Mandala wrote:
> >
> > > You do indeed remember correctly, gpsd is in buildroot but it's quite
> > > out of date, it's at 2.10 and gpsd is now at 2.25 or 2.26. There is a
> > > critical update in the latest gpsd for the gumstix, so it should be
> > > updated. The gumstix does not have a battery backed up RTC. The older
> > > versions of gpsd rely on an RTC if the gps is running in NEMA mode fo=
r
> > > the year. The newer versions of gpsd no longer require the RTC to
> > > function correctly.
> >
> > Just upped gpsd to 2.25 (latest) in subversion.  Looks like it wants c
> > ++ for libgpsmm, so I just hacked the Makefile to not include that in
> > the libgps.a so as to not require c++.
> >
>=20
> Super that should help some folks quite a lot. Took me a while to figure
> out what was going on, and then more time to get to the correct people
> at the gpsd team.
>=20
> > > Don't forget any database that uses transactions will do a "lot" of
> > > writing and deleting per each record written to the db. Unless you ar=
e
> > > using a hard disk you are going to use your flash far more then you
> > > suspect. Depending upon your design you might be better off using
> > > an in
> > > memory database or linked list, and an in-line battery and watch the
> > > incoming power and only write to the flash when the incoming power is
> > > lost.
> >
> > Easiest way to do this might be to run the DB out of /tmp or some
> > other in-core FS and then if you detect power failure, just shut down
> > the app and copy the DB file(s) to flash then.  If you're writing a
> > lot of data (ie more than RAM will allow, or would fit in the gumstix
> > flash anyway), then you can write to a CF or MMC flash card -- those
> > generally do very good wear-leveling in hardware, so even with very
> > frequent reads/writes (like a block per second), you can still
> > theoretically get hundreds of years MTBF.
> >
>=20
> Problem is a db with transactions on can really do some damage depending
> upon what types of transactions are done. I too archive gps settings and
> the good thing is they are small, usually less then 100 bytes per
> record. Put them in memory in a linked list and then write them out when
> necessary or when power is lost solves the problem of the wearing out
> the flash too soon.
>=20
> Cheers,
>=20
> David
>=20
> > C
> >
> >
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> --
> David Mandala <davidm at them dot com>
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>=20
>=20
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>=20


--=20
Dan Taylor

Software Development Engineer, JTL Systems Ltd
PhD Student, Reading University, UK

http://www.logicalgenetics.com