Hi,
On 05/ 5/11 03:02 PM, Edward Ned Harvey wrote:
From: Garrett D'Amore [mailto:garr...@nexenta.com]
We have customers using dedup with lots of vm images... in one extreme
case they are getting dedup ratios of over 200:1!
I assume you're talking about a situation where there is an initial VM image,
and then to clone the machine, the customers copy the VM, correct?
If that is correct, have you considered ZFS cloning instead?
When I said dedup wasn't good for VM's, what I'm talking about is: If there is data
inside the VM which is cloned... For example if somebody logs into the guest OS and then
does a "cp" operation... Then dedup of the host is unlikely to be able to
recognize that data as cloned data inside the virtual disk.
ZFS cloning and ZFS dedup are solving two problems that are related, but
different:
- Through Cloning, a lot of space can be saved in situations where it is
known beforehand that data is going to be used multiple times from multiple
different "views". Virtualization is a perfect example of this.
- Through Dedup, space can be saved in situations where the duplicate nature
of data is not known, or not known beforehand. Again, in virtualization
scenarios, this could be common modifications to VM images that are
performed multiple times, but not anticipated, such as extra software,
OS patches, or simply man users saving the same files to their local
desktops.
To go back to the "cp" example: If someone logs into a VM that is backed by
ZFS with dedup enabled, then copies a file, the extra space that the file will
take will be minimal. The act of copying the file will break down into a
series of blocks that will be recognized as duplicate blocks.
This is completely independent of the clone nature of the underlying VM's
backing store.
But I agree that the biggest savings are to be expected from cloning first,
as they typically translate into n GB (for the base image) x # of users,
which is a _lot_.
Dedup is still the icing on the cake for all those data blocks that were
unforeseen. And that can be a lot, too, as everone who has seen cluttered
desktops full of downloaded files can probably confirm.
Cheers,
Constantin
--
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