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Re: off-site backup

On Mon, Oct 16, 2006 at 01:50:15PM -0400, dtutty@porchlight.ca wrote:
> Does anyone know what the limit is?  10 years at monthy is 120 cycles;
> 10 years at weekly is 520 cycles, 10 years at daily is 3,650.

Different types of flash memory have different cycle counts.

> Does the cycle limit apply to the whole device or does a section get
> worn out and the capacity just shrink?  E.g. if I have my /etc/
> directory copied to it and one file in /etc/ changes, does changing that
> one file reduce the lifespan of the drive as a whole?

The capacity generally does not shrink.  Eventually you may simply wear
out enough sectors that it runs out of spares, and can no longer write
to enough sectors.  If it is one of the dumber ones that don't wearh
level very well you will simply get unwriteable sectors.  Since many
filesystems (especially FAT) have certain sectors they write to on every
update, that area has a tendancy to die first, leaving an essentially
unusuable flash disk.  You might be able to partition it with a
partition starting slightly in on the flash drive, to "shrink" the
drive, and move the filesystem index to a new area that wasn't used as
much before.

Make sure NOT to mount with -o sync on flash drives.  Some distributions
mistakenly used -o sync on all removeable media, which was very bad for
performance and also the lifetime of flash disks.

> I don't know how a memory chip gets translated into a 'drive'.  Is it
> like a HDD with spare sectors?

Depends on the firmware/controller in the device.  Some are very dumb
and do essentially one to one mapping.  Others are smart and do wear
leveling across the whole flash chip, so that anything that changes
often will be moved around, and stuff that never changes will be moved
to the more used sectors to let the less used sectors get some use.

Len Sorensen

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