Messages in this thread | | | Date | Wed, 11 Nov 1998 02:09:16 +0100 (CET) | From | MOLNAR Ingo <> | Subject | Re: New Solaris 7 features |
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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 (http://www.kernel.org/pub/linux/daemons/raid/alpha/), 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' device.
-- mingo
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