On Feb 18, 2006, at 11:44 PM, Bruce Black wrote:
Let me elaborate on what Ron said.
There are essentially 3 technologies out there for "instant"
replication of data.
1) Snapshot, in the IBM RVA and the StorageTek(Sun) disks, was
truly instant, since the architecture allowed data to be copied by
just copying pointers in memory. I won't go into all the gory
details, but essentially it created two pointers, so that two
different tracks (on the same or different disks) pointed to the
same track image on the real disks. If one of those two "tracks"
was updated, its data was written to a new location and its pointer
updated, the other still pointed to the unupdated data. 2) constant
mirroring, such as EMC Timefinder/Mirror BCV and Hitachi
ShadowImage, keep a continously updated mirror of a given disk
volume. When you want to create a point-in-time image of the
volume, you 'split" the mirror and it instantly becomes a copy.
3) instant copy - like Flashcopy and EMC Snap, copy datasets and
volumes by establishing a background session, which takes only
seconds. The copy can be treated as if it was instantly completed,
even though the control unit is still copying data in the
background. Updated tracks are copied immediately, before the
update is allowed, to preserve the original content. Unupdated
tracks may or may not be copied (an option); if a track on the
target is accessed and it has not been copied, the track from the
original disk is fetched.
BTW, Hitachi ShadowImage has a version of (3) for full-vol only.
They support FlashCopy for dataset copies.
---------SNIP--------------
Bruce, (and Ron):
Thanks for the mini tutorial.
I guess if everyone is happy with instant then who am I to say not
quite. Especially when the volume is X many miles away.
Locally I guess its close enough for most people to be happy with.
Ed
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