On Wed, Jan 4, 2012 at 4:55 AM, Swapnil Gaikwad <swapnilgaik72@xxxxxxxxx> wrote:
> If we store snapshot on other partition and assign new inode to each file's
> snapshot. Then datablocks of that partition where we store snapshot is
> wasted ? If not how?
That's effectively what a device mapper snapshot does.
Since only _changed_ blocks (inodes, pointer blocks, and data blocks)
are maintained in the snapshot volume, the snapshot volume can be much
smaller than the original volume.
I haven't studied the code, but obviously they have something like:
struct {
boolean virgin;
int alt_location;
} map[max_blocks];
So a read from the snapshot is just:
if (map[block].virgin)
read(primary_volume, block)
else
read(snapshot_volume,map[block].alt_location)
And write to the primary volume becomes
if (map[block].virgin) {
alt_loc=alloc_block_in_snapshot()
map[block].alt_location = alt_loc
map[block].virgin = false
copy(block,alt_loc)
}
write(primary_volume, block)
Then when your snapshot volume fills up, you have to discard it
because it can't hold anymore changes. So if your snapshot volume is
only 10% of the size of the primary volume, you can only support
having 10% of the original volume written to.
Greg
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