Re: [libvirt] [PATCH 3/4] Fix QEMU save/restore with block devices

On Thu, Apr 22, 2010 at 11:36:06AM +0100, Daniel P. Berrange wrote: > On Wed, Apr 21, 2010 at 11:02:33PM +0200, Daniel Veillard wrote: > > On Wed, Apr 21, 2010 at 05:56:12PM +0100, Daniel P. Berrange wrote: > > > The save process was relying on use of the shell >> append > > > operator to ensure the save data was placed after the libvirt > > > header + XML. This doesn't work for block devices though. > > > Replace this code with use of 'dd' and its 'seek' parameter. > > > > > > The qemuMonitorMigateToCommand() monitor API is used for both > > > save/coredump, and migration via UNIX socket. We can't simply > > > switch this to use 'dd' since this causes problems with the > > > migration usage. Thus, create a dedicated qemuMonitorMigateToFile > > > which can accept an filename + offset, and remove the filename > > > from the current qemuMonitorMigateToCommand() API > > > > > > * src/qemu/qemu_driver.c: Switch to qemuMonitorMigateToFile > > > for save and core dump > > > * src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h, > > > src/qemu/qemu_monitor_json.c, src/qemu/qemu_monitor_json.h, > > > src/qemu/qemu_monitor_text.c, src/qemu/qemu_monitor_text.h: Create > > > a new qemuMonitorMigateToFile, separate from the existing > > > qemuMonitorMigateToCommand to allow handling file offsets > > > --- > > > src/qemu/qemu_driver.c | 172 +++++++++++++++++++++++------------------- > > > src/qemu/qemu_monitor.c | 28 ++++++-- > > > src/qemu/qemu_monitor.h | 9 ++- > > > src/qemu/qemu_monitor_json.c | 35 ++++++++- > > > src/qemu/qemu_monitor_json.h | 9 ++- > > > src/qemu/qemu_monitor_text.c | 35 ++++++++- > > > src/qemu/qemu_monitor_text.h | 9 ++- > > > 7 files changed, 201 insertions(+), 96 deletions(-) > > > > > [...] > > > > > > - if (virAsprintf(&dest, "exec:%s >>%s 2>/dev/null", argstr, safe_target) < 0) { > > > + if (virAsprintf(&dest, "exec:%s | dd of=%s seek=%llub", > > > + argstr, safe_target, offset) < 0) { > > > > hum %llu will be converted to the value and then 'b' is appended. But > > my reading is that 'b' means block i.e. 512 bytes, and what we need is > > skeep to offset bytes, i.e. use seek=%lluc since 'c' means characters, > > or am I mistaken ? > > Hmm, yes I think you are correct. Oddly QEMU was happy enough restoring > from this. I assume dd will just push at the end if the offset is greater than the file size instead of creating a sparse file, so we end up with what we expect :-)