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

[Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>]

> That means if guest linux is executing "yum install kernel" when
> creating snapshot, the vm created from this snapshot might be not
> bootable.

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.  

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.

Ian

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
> 
> 
> 
> 
> >
> > Daniel
> >
> > > How does XCP make sure this snapshot is usable,say, virtual disk
> > > metadata is consistent?
> > >
> > > Thanks
> > > - 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
> on
> > > > > regardless how big the physical size of this node?
> > > >
> > > > Yes: it's always good to coalesce the chain to improve access
> performance.
> > > >
> > > > > 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
> coalesced).
> > > >
> > > > 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
> coalesce
> > > >
> > > > 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...
> > > >
> > > > Ian
> > > >
> > > > >
> > > > > - 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
> space
> > > > > >  efficient. > > > But I still have below concern. > > There is
> one more
> > > > > >  vhd chain when creating snapshot, if I creates 16 > snapshots,
> there
> > > > > >  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
> block,
> > > > > >  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
> block
> > > > > >  device handler whether a block has been fully allocated. Once
> the
> > > > > >  block is fully allocated (all logical 2MB written) the block
> handler
> > > > > >  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
> offset.
> > > > > >  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
> until
> > > > > >  it detects a node that is allocated with hopefully the BATmap
> value
> > > > > >  set. The worst case is a random disk write workload which
> causes the
> > > > > >  disk to be fragmented and partially allocated. Every read or
> write
> > > > > >  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
> several
> > > > > >  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.
> Coalesceable
> > > > > >  work is defined as:
> > > > > >
> > > > > > 'a hidden child node that has no siblings'
> > > > > >
> > > > > > Hidden nodes are non-leaf nodes that reside within a chain. When
> the
> > > > > > snapshot leaf node is deleted therefore, it will leave redundant
> links
> > > > > > in the chain that can be safely coalesced. You can kick off a
> coalesce
> > > > > > by issuing an SR scan, although it should kick off automatically
> within
> > > > > > 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
> that
> > > > > > deleting snapshot nodes does not always mean that there is
> coalescable
> > > > > > 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
> deleting
> > > > > > > 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
> almost
> > > > > > 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
> coalesced
> > > > > > in order to maintain data integrity.
> > > > > >
> > > > > > Thanks,
> > > > > > Julian
> > > > > > >
> > > > > > > - Anthony
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > >
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> > > > > >
> > > > >
> > > > >
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