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SubjectRe: New Solaris 7 features

On Tue, 10 Nov 1998, Malcolm Beattie wrote:

> While I'm here: an idea for providing a snapshot capability to any
> block device (allowing you to snapshot then backup any fs you like).

this is a planned feature of the MD based LVM personality i'm working on.
The prototype of the LVM is already in the latest MD patch
(, the snapshot
feature will probably show up in the next release. (in a few days)

> A fake block device /dev/snap which you connect to the
> device-to-be-snapshotted on one side and to a spare block device on
> the other side (either a spare partition or a /dev/loop on a file).

the snapshotted device does a 'copy-out' to a 'store old contents' device
for every (block) write after the snapshot time. The 'old contents' device
is some scratch partition or an on-demand-storage LVM device. (ie. if the
device runs out of space, it will not fail, it can self-extend itself)

> snap keeps a dirty bitmap and a sparse block structure on the spare
> block device. Writes to snap mark the block in the dirty bitmap and
> store the original block in the sparse structure. Reads from snap
> check the dirty bitmap and read from the original or the sparse block
> map.

we do not want to write to the 'snap'. We want to use the 'snapshot
device' to carry a snapshot for some limited (or unlimited) time, eg. for
backup purposes. This thing is completely transparent to the 'normal'

-- mingo

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