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Re: [Xen-API] How snapshot work on LVMoISCS SR

Ian Pratt wrote:
That means if guest linux is executing "yum install kernel" when
creating snapshot, the vm created from this snapshot might be not

Because xen issues write completions to the guest only when IO is completed, 
the snapshot will at least be crash consistent from a filesystem point of view 
(just like a physical system loosing power).

Linux doesn't have a generic mechanism for doing higher-level 'freeze' operations (see Windows VSS) so there's no way to notify yum that we'd like to take a snapshot. Some linux filesystems do support a freeze operation, but it's not clear this buys a great deal.
Ack. Without application signalling (as provided by VSS) it's unclear whether there's any real benefit since the application data may still be internally inconsistent.

FYI - for windows VMs XCP includes a VSS quiesced snapshot option (VM.snapshot_with_quiesce) which utilises the agent running in the guest as a VSS requestor to quiesce the apps, flush the local cache to disk and then trigger a snapshot for all the VMs disks.

- Julian
99 times out of 100 you'll get away with just taking a snapshot of a VM. If 
you're wanting to use the snapshot as a template for creating other clones 
you'd be best advised to shut the guest down and get a clean filesystem though. 
Any snapshot should be fine for general file backup purposes.


PS: I'd be surprised if "yum install kernel" didn't actually go to some lengths 
to be reasonably atomic as regards switching grub over to using the new kernel, otherwise 
you'd have the same problem on a physical machine crashing or losing power.

- Anthony


How does XCP make sure this snapshot is usable,say, virtual disk
metadata is consistent?

- Anthony

On Tue, 2010-01-26 at 13:56 -0800, Ian Pratt wrote:
I still have below questions.

1. if a non-leaf node is coalesce-able, it will be coalesced later
regardless how big the physical size of this node?
Yes: it's always good to coalesce the chain to improve access
2. there is one leaf node for a snapshot, actually it may be
empty, does
it exist only because it can prevent coalesce.
Not quite sure what you're referring to here. The current code has a
limitation whereby it is unable to coalesce a leaf into its parent, so
after you've created one snapshot you'll always have a chain length of 2
even if you delete the snapshot (if you create a second snapshot it can be
Coalescing a leaf into its parent is on the todo list: its a little
bit different from the other cases because it requires synchronization if
the leaf is in active use. It's not a big deal from a performance point of
view to have the slightly longer chain length, but it will be good to get
this fixed for cleanliness.
3. a clone will introduce a writable snapshot, it will prevent
A clone will produce a new writeable leaf linked to the parent.  It
will prevent the linked snapshot from being coalesced, but any other
snapshots above or below on the chain can still be coalesced by the
garbage collector if the snapshots are deleted.
The XCP storage management stuff is pretty cool IMO...


- Anthony

On Tue, 2010-01-26 at 02:34 -0800, Julian Chesterfield wrote:
Hi Anthony,

Anthony Xu wrote: > Hi all, > > Basically snapshot on LVMoISCSI
SR work
 well, it provides thin > provisioning, so it is fast and disk
 efficient. > > > But I still have below concern. > > There is
one more
 vhd chain when creating snapshot, if I creates 16 > snapshots,
 are 16 vhd chains, that means when one VM accesses a > disk
block, it
 may need to access 16 vhd lvm one by one, then get the > right
 it makes VM access disk slow. However, it is > understandable,
it is
 part of snapshot IMO. >   The depth and speed of access will
depend on
 the write pattern to the disk. In XCP we add an optimisation
called a
 BATmap which stores one bit per BAT entry. This is a fast
lookup table
 that is cached in memory while the VHD is open, and tells the
 device handler whether a block has been fully allocated. Once
 block is fully allocated (all logical 2MB written) the block
 knows that it doesn't need to read or write the Bitmap that
 corresponds to the data block, it can go directly to the disk
 Scanning through the VHD chain can therefore be very quick,
i.e. the
 block handler reads down the chain of BAT tables for each node
 it detects a node that is allocated with hopefully the BATmap
 set. The worst case is a random disk write workload which
causes the
 disk to be fragmented and partially allocated. Every read or
 will therefore potentially incur a bitmap check at every level
of the
 chain. > But after I delete all these 16 snapshots, there is
still 16
 vhd chains, > the disk access is still slow, which is not
 understandable and > reasonable, even though there may be only
 KB difference between > each snapshot, >   There is a mechanism
in XCP
 called the GC coalesce thread which gets kicked asynchronously
 following a VDI deletion event. It queries the VHD tree, and
 determines whether there is any coalescable work to do.
 work is defined as:

'a hidden child node that has no siblings'

Hidden nodes are non-leaf nodes that reside within a chain. When
snapshot leaf node is deleted therefore, it will leave redundant
in the chain that can be safely coalesced. You can kick off a
by issuing an SR scan, although it should kick off automatically
30 seconds of deleting the snapshot node, handled by XAPI. If
you look
in the /var/log/SMlog file you'll see a lot of debug information
including tree dependencies which will tell you a) whether the
GC thread
is running, and b) whether there is coalescable work to do. Note
deleting snapshot nodes does not always mean that there is
work to do since there may be other siblings, e.g. VDI clones.
is there any way we can reduce depth of vhd chain after
snapshots? get VM back to normal disk performance.

The coalesce thread handles this, see above.
And, I notice there are useless vhd volume exist after
deleting snap
shots, can we delete them automatically?

No. I do not recommend deleting VHDs manually since they are
certainly referenced by something else in the chain. If you
delete them
manually you will break the chain, it will become unreadable,
and you
potentially lose critical data. VHD chains must be correctly
in order to maintain data integrity.

- Anthony

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