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snap shot space

Posted on 2013-01-18
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I have seen many storage devices require 25% or somekind of % space for snapshot.
i am wondering how it is possible when i create 10GB VD and he snapshot space only 2.5GB.
is not it suppose to be 1:1ratio?
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Question by:pdsmicro
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by:nobus
ID: 38795923
what exactly do you you mean with snapshot space??
can you explain what this is about??
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by:Gerald Connolly
ID: 38796492
A Clone would be 1:1 space but a Snapshot only holds the blocks changed, thats why they normally only take up a small proportion of the volume they are a snapshot of. An old rule of thumb is that 80% of writes goto 20% of the disk.

Snapshots can either be Copy-on-write  or Redirect-on-write, COW means the original data is stored in the snapshot, ROW means the new data is stored in the snapshot
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by:Paul Solovyovsky
ID: 38796511
It depends on the hardware/software. Are you talking about consumer or enterprise grade storage?  What exactly are you looking to accomplish and does it involve specific application or just file server data?
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by:pdsmicro
ID: 38805123
This is an enterprise storage devices.
I read dell equalllogic tech notes and it says that we allocate 25% of volume space for snapshot.
NetApp says % for snapshoto.
Jetstore says 2:1 for snap shot.

When we create 1TB space for volume the we give 25GB for snapshot.
how that 1TB data is being converted to 250GB?
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Gerald Connolly earned 2000 total points
ID: 38805226
Its about layers of virtualisation. When you create a volume (say a E: drive) on a LUN of a RAIDset, you are creating a virtual disk whose virtual blocks are mapped onto the Physical Spindles used in the volume. These virtual blocks can be visualised as a set of pointers.

When you create a snapshot of a volume, aside from any preallocation of space all that is required is the space required for a copy of the pointers plus any header info required ie a tiny amount of space, probably MBytes worth of space depending on the size of the original volume. No data space needs to be allocated although most implementations will preallocate some space to speed up the process.

So now you have two sets of pointers the Original set (orgset) and the Snapshot set (snpset) both pointing at the original data.

Now when you now try and write to the original Volume it ultimately ends up as a write to a block (or blocks) and before the write happens, space is allocated in the snapshot (either by using some of the pre-allocated space or by grabbing some of the spare space on the RAIDsystem.

In a CoW implementation -   The blocks in question are then copied into the snapshot space and they pointer for the snpset is modified to point to this copied data, the write completes to the original volume, but the original pointer is not changed.

In a RoW implementation - The write completes to the snapshot space and orgset is modified to point to the new space, but the snpset is not changed.

If you only write to 25% of the disk the snapshot space will only be 25% of the size of the original volume, but if you write to 100% of the original volume then the snapshot will take up 100% of the space taken up by the original.
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by:pdsmicro
ID: 38808374
thank you
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