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xen-api
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
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.
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.
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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