Re: [Libguestfs] virt-sparsify and iSCSI

On Mon, Jan 29, 2018 at 03:04:39AM +0200, Shmuel Melamud wrote: Note there are two quite different virt-sparsify modes. I'm going to assume here you're talking about ‘virt-sparsify --in-place’. In-place works by doing either fstrim (or in some cases blkdiscard) on all the filesystems in the disk image. There are a couple of things to say about this: (a) There's a very complex interaction between the libguestfs appliance kernel, qemu, the host kernel and the actual backing store to make this work. And (b) fstrim/blkdiscard is a voluntary hint, which any layer in the storage system may ignore without even a warning. So it's entirely possible this will work for some storage systems and then silently stop working because some element in the stack changes. In this case let's assume it's iSCSI which is at fault. I believe qemu will see a /dev/sdX device and will issue ‘ioctl (fd, BLKDISCARD...)’: https://git.qemu.org/?p=qemu.git;a=blob;f=block/file-posix.c;h=36ee89e940... You can probably strace qemu to see if it's doing this. As you can see from the rest of the file-posix.c file there are a lot of different cases depending on what the kernel and storage system report. Also it depends on various SCSI flags reported by the device. These are contained in the SCSI "VPD" data. It might be interesting to run sg_inq and sg_vpd (from sg3_utils) on the device: $ sudo sg_inq -p 0xb0 /dev/sda $ sudo sg_vpd -p 0xb2 /dev/sda The LBPRZ & various unmap settings are all relevant. See also: https://rwmj.wordpress.com/2014/03/11/blkdiscard-blkzeroout-blkdiscardzer... Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/