As part of a new virt-v2v feature, I've been thinking about how to write
data to an arbitrary block device in the appliance. I need to be able to
write arbitrary chunks of data to specific places on the device. This
will need a new API, as guestfs_pread can't open a block device. While
I'm at it, I'd like to create a new family of APIs which operate on a
file handle:
int guestfs_open_device(g, path);
Open a block device. Return a handle.
int guestfs_open_file(g, path);
Open a file from the guest's filesystem. Return a handle.
int guestfs_close_handle(g, handle);
Close a file handle.
ssize_t guestfs_hpread(g, handle, buf, size, offset);
ssize_t guestfs_hread(g, handle, buf, size);
ssize_t guestfs_hpwrite(g, handle, buf, size, offset);
ssize_t guestfs_hwrite(g, handle, buf, size);
Wrappers round pread(), read(), pwrite() and write() which operate on
a handle.
ssize_t guestfs_hseek(g, handle, offset, whence);
Wrapper round lseek().
int guestfs_htruncate(g, handle, length);
Wrapper round ftruncate().
In terms of the API, the handle would be an opaque int. In practise it
would be a file descriptor owned by guestfsd.
I believe these APIs could result in a performance improvement over
opening a file repeated. However, I have no data to back this up at present.
Aside from any potential performance improvement I believe these APIs
are a better interface for the developer when doing low level file
operations in the guest.
Comments before I code this up?
Thanks,
Matt
--
Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team
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On Thu, Aug 19, 2010 at 05:34:38PM +0100, Matthew Booth wrote:
Aside from any potential performance improvement I believe these
APIs are a better interface for the developer when doing low level
file operations in the guest.
Comments before I code this up?
All seems pretty reasonable. I have no specific comments.
Rich.
--
Richard Jones, Virtualization Group, Red Hat
http://people.redhat.com/~rjones
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