Andreas, As I understand, the CF cards already implement wear leveling algorithms. Each time a block is updated / written a new block in the CF is used.
Jordi. 2010/7/13 Andreas Delleske <[email protected]> > Dear Jordi, > > thank you for your friendly reply! > > > We use adjtimex + ntpd in order to have a ALIX3D2 synchronized. I > > wrote how we configured it in > > > http://code.google.com/p/wfrog/wiki/InstallOn_Alix3D2_voyageLinux0_6_2#Clock_sync > > Thanks, I've found that page already.. :-) > > I found out that I could install adjtimex package as soon as I changed > the apt-sources to the german server.. > > Now the only buggy package that I need is ssmtp (smtp forwarder to a > smarthost), but i works when ignoring errors when installing. > > > I set up a read-write partition for the database and write a record > > each 5 min. So far the ALIX 3 D 2 with a 1Gb SLC Compact Flash has > > been working for half a year perfectly. We'll have to wait a couple of > > years to know how long the CF last!!! > > Yes.. > > If we think about wear leveling, there is a lot of options: > > Here, we will have only tens to hundreds of write updates per day, not > thousands, and I have 4 GByte Flash memory, only 400 MB of which are > used so far. The part where the changes appear (the database) is only > 3 MB now and will 10 MB in some years. > > If we assume that every change rewrites 100 kByte (much less in the > database's data, but with every update one or three flash blocks have > to be rewritten..) we rewrite max. 10 MB per day = 3,65 GBytes per > year - within one year, the free space on that disk is rewritten only > once (statistically) that is, if we have a proper wear-leveling > algorithm. If every flash block can be rewritten only 10.000 times, > can we think of 10.000 years? I think that would be enough. :-) Or did > I go wrong? > > Does th CF card "know" which blocks are free and use them > automatically with a round-robin-method? If the CF knows nothing about > filesystems, it would be much less: Assume we write always in the same > 10 MB, the CF will last 10.000 days = 27,3 years. Shouldnt even that > be enough? > > I see that if we have one small table in the DB which gets rewritten > always in the same spot, things get worse: 10.000 operations would be > accomplished after only a hundred days... maybe it is crucial to put > only that table in RAM and let the rest of the database grow slowly > without any concern? > > However, it would be great to be able to check the "sanity" of the > CF-disk like with the smartmontools and in some years just swap the > disk (they are inexpensive anyway). > > Or we could move that table from time to time to another location on > the flash ("Manual" wear-leveling)? Lets say we create a copy of the > full database every day and leave to old copy unattended until the > free space is filled up so with every update a new spot is used? Then, > with 3,5 GByte free space divided by 10 MByte a day we'd fill the disk > within a year - but could we redo that 10.000 times? That would be > enough as well. > > -- > Kind regards > Andreas >
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