[Fedora-livecd-list] Persistance Overlay
Douglas McClendon
dmc.fedora at filteredperception.org
Thu Jun 5 06:17:23 UTC 2008
Jeremy Katz wrote:
> On Wed, 2008-06-04 at 19:12 +0100, Pedro Silva wrote:
>> So, how does the persistance work:
>>
>> - Persistance is created with livecd-iso-to-disk, if I run it again
>> without the overlay flag, previous persistance overlay is deleted?
>
> Yes. Also, the overlay is specific to the *exact* filesystem image that
> it is created for.
>
> ... and for the gory details. The way that persistence is implemented
> is that the persistence file is used as the backing store for the device
> mapper snapshot that we build on top of the "base" filesystem image.
> This means that it contains just changed blocks from the base image and
> that changes continue to build up over time, never reusing blocks from
> the snapshot.
This is not actually true. It's not that it never reuses blocks from
teh snapshot, it's that it doesn't try to.
If say you have a 256MB overlay, half used, and half of that wasted
(e.g. you copy two 128MB files to /home, then delete one of them). Then
currently there will be about 2G of free blocks on 4G of rootfs, and
128MB of those currently in a wasted state on the overlay. Assuming
ext3 is roughly random in its choice of free block to use, that means
that each new block written has about a 1/16 chance of actually
reclaiming overlay space, instead of using more space. Theoretically if
you have a 4G overlay, you'll never run out of space, though at some
point you'll run into the issues of why you didn't just install fedora
to a 4G partition on that device to start with.
-dmc
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