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xen-api
Re: [Xen-API] How snapshot work on LVMoISCS SR
Hi Anthony,
Ian already covered most of the queries, but just to add some more detail:
Anthony Xu wrote:
Hi Julian/Dave,
Thanks for your detailed explanation,
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?
Correct. The background coalesce task will always coalesce nodes if
there is work that can be done. Note that there are out of space
conditions where this may be tricky, so the algorithm always tries to
coalesce the smallest differencing disk in a chain first.
2. there is one leaf node for a snapshot, actually it may be empty, does
it exist only because it can prevent coalesce.
It is true that the presence of a leaf node for a snapshot does prevent
coalesce from occuring, however the original motivation for providing a
leaf node was to enable "writeable snapshots". Some VMs, windows in
particular require that any disk attached to the OS be writeable. When a
disk is presented to the OS it scribbles a disk signature at the head of
the disk, and for some versions (XP and 2k3 I believe), you will
actually see a blue screen if the disk is read-only, i.e. writes are
blocked at the block device driver level.
3. a clone will introduce a writable snapshot, it will prevent coalesce
Correct.
- Julian
- 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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