[Fedora-livecd-list] Persistance Overlay

[Douglas McClendon]

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