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    SubjectRe: LVM vs. Ext4 snapshots (was: [PATCH v1 00/30] Ext4 snapshots)
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    On Fri, Jun 10, 2011 at 1:11 PM, Joe Thornber <thornber@redhat.com> wrote:
    > On Fri, Jun 10, 2011 at 11:01:41AM +0200, Lukas Czerner wrote:
    >> On Fri, 10 Jun 2011, Amir G. wrote:
    >>
    >> > CC'ing lvm-devel and fsdevel
    >> >
    >> >
    >> > On Wed, Jun 8, 2011 at 9:26 PM, Amir G. <amir73il@users.sourceforge.net> wrote:
    >> > For the sake of letting everyone understand the differences and trade
    >> > offs between
    >> > LVM and ext4 snapshots, so ext4 snapshots can get a fair trial, I need
    >> > to ask you
    >> > some questions about the implementation, which I could not figure out by myself
    >> > from reading the documents.
    >
    > First up let me say that I'm not intending to support writeable
    > _external_ origins with multisnap.  This will come as a suprise to
    > many people, but I don't think we can resolve the dual requirements to
    > efficiently update many, many snapshots when a write occurs _and_ make
    > those snapshots quick to delete (when you're encouraging people to
    > take lots of snapshots performance of delete becomes a real issue).
    >

    If I understand this article correctly:
    http://people.redhat.com/mpatocka/papers/shared-snapshots.pdf
    It says that _external_ origin write updates can be efficient to readonly
    (or not written) snapshots.

    Could you not support readonly snapshots of an _external_ origin?

    You could even support writable snapshots, that will degrade write
    performance to origin temporarily.
    It can be useful, if one wants to "try-out" mounting a temporary
    writable snapshot, when the origin is not even mounted.
    After the "try-out", the temporary snapshot can be deleted
    and origin write performance would go back to normal.

    Is that correct?

    Amir.
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