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Subject: Re: [linux-lvm] relocate on write for snapshot
From: Ming Zhang <mingz@ele.uri.edu>
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To: LVM general discussion and development <linux-lvm@redhat.com>

On Fri, 2004-11-19 at 12:19, Nils Juergens wrote:
> On Fri, 19.11.04, Ming Zhang <mingz@ele.uri.edu> wrote:
> 
> > instead of copy old one to snapshot, overwrite old one with new one, 2
> > writes and 1 reads. it is possible that write new data to a usused
> > location directly.
> 
> Since you have to delete the snapshot (and I don't think they live very long
> on most systems) you don't gain anything (because you have copy the changes
> back from the snapshot), but you make the cases of more than one snapshot
> and (possibly) read-write snapshots a lot harder, maybe not in CPU or IO,
> but certainly in code complexity, which is a thing you never do to gain
> a bit of performance.
> 
on some systems that mainly for disaster recovery purpose, there will be
a lot of snapshots available for a lv. this can make the recovery much
easier.

> > i know later remove a snaphot will be a little trouble, but there must
> > be some way to get around it.
> 
> Yes, you have to do the work you saved a couple of minutes earlier :)
> 
a dedicated snapshot merge process can solve this.

> just my EUR 0.02,
thanks. discussion always can make thing clarified.


> Nils