On Fri, 2011-04-08 at 11:13 -0600, Eric Blake wrote:
On 04/08/2011 07:31 AM, Daniel P. Berrange wrote:
On Thu, Apr 07, 2011 at 04:31:59PM -0500, Adam Litke wrote:
/* + * Disk Streaming + */ +typedef enum { + VIR_STREAM_DISK_ONE = 1, /* Stream a single disk unit */ + VIR_STREAM_DISK_START = 2, /* Stream the entire disk */ + VIR_STREAM_DISK_STOP = 4, /* Stop streaming a disk */ +} virDomainStreamDiskFlags;
Using flags to combine two separate tasks into one single API is rather unpleasant. As raised in the previous patch, the API should also be taking a offset+length in bytes, then there is no need for a special case transfer of an individual sector.
Taking all the previous points together, I think the API needs to look like this:
typedef enum { /* If set, virDomainBlockAllocate() will return immediately * allowing polling for operation completion & status */ VIR_DOMAIN_DISK_ALLOCATE_NONBLOCK, } virDomainBlockAllocateFlags;
It seems like adding one more flag would also make this API useful for supporting the converse operation: if we have a disk that is currently allocated, but we either know that a block is all 0s (or don't care about the data in that block if it was not all 0s), it would be nice to request punching a hole (for file-backed images residing on a file system and kernels new enough to do that) and/or truncate back to a smaller (thinly-provisioned) allocated size (which should work for both file-backed and lvm-backed disk images).
I agree that this could be a good future extension of the API and further justifies the use of offset _and_ length parameters. However, I'd prefer not to consider this part at the moment since I am not aware of a hypervisor that plans to implement it.
Meanwhile, I know that GNU coreutils has been working on an API for efficiently getting FIEMAP data from files; part of this effort needs to involve migrating that code into gnulib so that libvirt can indeed provide an API that enumerates sections of a disk image that are allocated vs. holes.
Why not just ask the hypervisor? Qemu's image format code is probably the most efficient place from which to gather this information. -- Thanks, Adam