On Thu, Jan 12, 2012 at 5:00 PM, Jim Klimov <jimkli...@cos.ru> wrote:

> While reading about zfs on-disk formats, I wondered once again
> why is it not possible to create a snapshot on existing data,
> not of the current TXG but of some older point-in-time?

It is not possible because the older data may no longer exist on-disk.  For
example, you want to take a snapshot from 10 txg's ago.  But since then we
have created a new file, which modified the containing directory.  So we
freed the directory block from 10 txg's ago.  That freed block is then a
candidate for reallocation.

Existence of old uberblocks in the ring buffer does not indicate that the
data they reference is still valid.  This is the reason that "zpool import
-F" does not always work.

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