2 basic LVM questions with RAMDISKs

Hank heskin at gmail.com
Mon Feb 7 07:25:10 CST 2011


Thank you all for your clarifications...  Looks like LVM does work in the
manner I had hoped.

I'll see about getting a verbose output of PVCREATE and report back.

-Hank


On Mon, Feb 7, 2011 at 7:03 AM, <David.Harding at brighthouse.co.uk> wrote:

>
> Sunil,
>
> No, I'm not saying that updates to the snapshot get written back to the
> original volume.
> I may have mis-understood your reply to Hank "For the second point any
> change in snapshot will not go to the original  LVM it’s vice versa any new
> changes in LVM will be going to the snapshot. "
> Hank's reason for using a snapshot was to backup the original LV. He was
> asking if he'd lose updates to the original LV when the snapshot was
> removed, your reply seemed to suggest that he would, where you said that
> "any new changes in LVM will be going to the snapshot".
> This is not the case.
> A snapshot will contain the original copies of updated extents. So, updates
> to the original LV will go to the original LV; how the affected extents
> looked before the update will go to the snapshot: Thus, when you backup the
> snapshot LV (for the purpose of backing up the original LV), you capture the
> original LV as it was when the snapshot was taken.
>
> From the snapshotintro:
> "LVM1 has read-only snapshots. Read-only snapshots work by creating an *exception
> table*, which is used to keep track of which blocks have been changed. If
> a block is to be changed on the origin, it is first copied to the snapshot,
> marked as copied in the exception table, *and then the new data is written
> to the original volume*.
>
> In LVM2, snapshots are read/write by default. *Read/write snapshots work
> like read-only snapshots*, with the additional feature that *if* data is
> written to the snapshot, that block is marked in the exception table as
> used, and never gets copied from the original volume."
>
> (my underlining).
>
> The answer to Hank's question is no, he will NOT lose updates when he
> removes the snapshot, as the updates go to the original LV.
> My apologies if I've mis-understood your reply.
>
>
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