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?
2. there is one leaf node for a snapshot, actually it may be empty, does
it exist only because it can prevent coalesce.
3. a clone will introduce a writable snapshot, it will prevent coalesce
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.
> > - Anthony
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