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Has anyone got information on how to create, manage and view the snapshots. I have created a snapshot of a volume but when trying to view from a windows client i cannot view anything as it wont allow me to initialize the disk. This is on the demo cd version 1.60 2006/04/14
Thanks
Gavin
Can you download the latest version from 8-28-2006 demo_73.20060825.169.b2146.iscsi-ent.iso The current one you are running is older. Also try to set the Snapshot to Read only in the Target Volume Manager. Below is the Snapshot functions.
SNAPSHOT:
This function allows you to define parameters of every snapshot.
You can set:
* number of snapshots for logical volumes in specified volume group
* logical volume (TGV), which the snapshot will be taken for.
* space reserved for the changes in file system while the snapshot is active - you
enter value as a percent of space reserved for snapshots
* schedule - the time of automatic creation of the snapshot, if inactive - only
manual snapshot activation is possible
* RO - the snapshot will be visible as a write protected disk
The Snapshot function of the server enables the system administrator to freeze the data content of the volume at a certain time. From this moment on, the users work on a virtual data volume, all changes to the volume are stored in a different partition. The storage of all changes is independent of the file-system - it takes place on block-level. Only when the snapshot is deactivated / removed the changes are permanently transferred to the actual data volume. Snapshots can be activated/deactivated manually or automatically.
NOTE:
Please be reasonable, when you are calculating the space reserved for snapshots. Please set as snapshot size as much space as you expect to change during active snapshot. E.g. when you are doing backup from snapshot which takes one hour please set this snapshot size to as much space that will be changed during one hour. The snapshot will become inactive if the content (data changed on logical volume) exceeds the snapshot capacity. You will not lose data in that case, just the dataset, which is virtual for the users at the moment and will be written to the data volume. The old dataset, which has been frozen with the snapshot, is not available any longer.
When you define the schedule, use only as many snapshots in the same time as really needed. A large count of active snapshots can slow down the system considerably.
Manual creating and removing of snapshots can be done in the maintenance -> snapshot menu.
LOCATION:
setup -> target manager -> volumes -> vg[nr] -> Function: Snapshots definition.
That last post is a little unclear on snapshot sizing. (It reads verbatim from the latest manual)
Lets try some examples here and see if someone can straighten me out.
I created a lv of 300gb and a snapshot volume of 300gb and attached it to the main lv.
Now since it is a block device, does it just do a block copy? Or does it do a point in time difference (delta)? If it is a delta, is the lv a delta of the snapshot or the otherway around?
Since I created a 300gb lvm and a 300gb snapshot, am I only able to get 1 snapshot? So if I decided to create a snapshot schedule that would take a snapshot on Sat nights when no one is using the system and just have it overwrite the previous snapshot, would that work?
Thanks
Last post was very clear on discussing the snapshot size as there was no follow up question from the response. This part is what is important about snapshot sizing that I will copy and past again. “Please set as snapshot size as much space as you expect to change during active snapshot. E.g. when you are doing backup from snapshot which takes one hour please set this snapshot size to as much space that will be changed during one hour.” Concerning the inserting of the manual, a lot of new customers do not have time to download the manual and this saves them time as you can see there was no reply so he understands.
Snapshot is based on the point in time difference (or known as Delta after the change) of the Logical Volume “The storage of all changes is independent of the file-system - it takes place on block-level” .
In your case if you have 300GB LV and you feel there will be changes during the time that you have specified that will exceed a certain amount lets say 30GB +or- during this time then you would set 10% -15% snapshot of the designated 300GB volume. Snapshots
Will not overwrite another snapshot only update after re-activating it again in the Maintenance > Snapshot function then it will update from dataset changes of the Logical Volume. Please read the manual and create test so you can see how this process works also remember to limit your snapshots as this will effect performence.
I also have some additional information on our Snapshot technology as it is based on
Copy-on-write (sometimes referred to as "COW") this is an optimization strategy used in computer programming. The fundamental idea is that if multiple callers ask for resources which are initially indistinguishable, you can give them pointers to the same resource. This fiction can be maintained until a caller tries to modify its "copy" of the resource, at which point a true private copy is created to prevent the changes becoming visible to everyone else. All of this happens transparently to the callers. The primary advantage is that if a caller never makes any modifications, no private copy need ever be created.
Copy-on-write finds its main use in virtual memory operating systems; when a process creates a copy of itself, the pages in memory that might be modified by either the process or its copy are marked copy-on-write. When one process modifies the memory, the operating system's kernel intercepts the operation and copies the memory so that changes in one process's memory are not visible to the other.
Another use is in the call function. This can be implemented by having a page of physical memory filled with zeroes. When the memory is allocated, the pages returned all refer to the page of zeroes and are all marked as copy-on-write. This way, the amount of physical memory allocated for the process does not increase until data is written. This is typically only done for larger allocations.
Copy-on-write can be implemented by telling the MMU that certain pages in the process's address space are read-only. When data is written to these pages, the MMU raises an exception which is handled by the kernel, which allocates new space in physical memory and makes the page being written to correspond to that new location in physical memory.
One major advantage of COW is the ability to use memory sparsely. Because the usage of physical memory only increases as data is stored in it, very efficient hash tables can be implemented which only use little more physical memory than is necessary to store the objects they contain.
The COW concept is also used in virtualization/emulation software such as Bochs, QEMU, and UML for virtual disk storage. This allows a great reduction in required disk space when multiple VMs can be based on the same hard disk image, as well as increased performance as disk reads can be cached in RAM and subsequent reads served to other VMs out of the cache