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

[Anthony Xu <anthony@xxxxxxxxx>]

It is clear now, thanks.

The other thing I'd like to do is how XCP handle disk cache inside VM
when creating a snapshot? I saw from Xencenter seem the VM is stopped
temporarily when creating a snapshot.

Does VM flush dirty disk cache when creating snapshot?

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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