Re: [libvirt] [PATCH] Allow saving QEMU libvirt state to a pipe

All, Currently the "virsh save" command opens a file to save a domain's XML and memory state, does the save, and then re-opens the file to simulate seeking to the beginning to update the header to indicate that the file is complete. For pipes this is not possible, attempting to re-open the pipe will not connect you to the same consumer. Seeking is also not possible on a pipe. Attached is a patch that detects if the file being written to is not seekable, and if so writes the completed header initially, then writes the normal data, and avoids re-opening the file. This is useful to me for saving a VM state directly to Ceph RBD images without having an intermediate file. I've tested this patch successfully (against v1.3.5) for both saving and restoring a domain: # *( fifo="$(mktemp -u)"; mkfifo "${fifo}" && virsh save one-0 "${fifo}" & cat "${fifo}" | rbd import - rbd/test1234 & wait; rm -f "${fifo}" )* Domain one-0 saved to /tmp/tmp.HK4hChiQqB # # *( fifo="$(mktemp -u)"; mkfifo "${fifo}" && rbd export rbd/test1234 - > "${fifo}" & virsh restore "${fifo}" & wait; rm -f "${fifo}" ) * Exporting image: 100% complete...done. Domain restored from /tmp/tmp.0YaUZ5Y2yT # *virsh list* Id Name State ---------------------------------------------------- 11 one-0 running # -- Roy Keene Knight Point Systems, LLC Service-Disabled Veteran-Owned Business 1775 Wiehle Avenue Suite 101 | Reston, VA 20190 c: 813-956-3808 f: 571-266-3106 www.knightpoint.com DHS EAGLE II Prime Contractor: FC1 SDVOSB Track GSA Schedule 70 SDVOSB: GS-35F-0646S GSA MOBIS Schedule: GS-10F-0404Y ISO 20000 / ISO 27001 Notice: This e-mail message, including any attachments, is for the sole use of the intended recipient(s) and may contain confidential and privileged information. Any unauthorized review, copy, use, disclosure, or distribution is STRICTLY prohibited. If you are not the intended recipient, please contact the sender by reply e-mail and destroy all copies of the original message.

diff --no-dereference -uNr libvirt-1.3.5.orig/src/qemu/qemu_driver.c libvirt-1.3.5-savepipe/src/qemu/qemu_driver.c --- libvirt-1.3.5.orig/src/qemu/qemu_driver.c 2016-05-28 11:47:33.000000000 -0500 +++ libvirt-1.3.5-savepipe/src/qemu/qemu_driver.c 2016-11-04 10:54:15.160805261 -0500

@@ -3080,6 +3080,7 @@ virQEMUSaveHeader header; bool bypassSecurityDriver = false; bool needUnlink = false; + bool canReopen = true; int ret = -1; int fd = -1; int directFlag = 0; @@ -3087,7 +3088,6 @@ unsigned int wrapperFlags = VIR_FILE_WRAPPER_NON_BLOCKING; memset(&header, 0, sizeof(header)); - memcpy(header.magic, QEMU_SAVE_PARTIAL, sizeof(header.magic)); header.version = QEMU_SAVE_VERSION; header.was_running = was_running ? 1 : 0; header.compressed = compressed; @@ -3109,12 +3109,32 @@ if (fd < 0) goto cleanup; + /* + * Determine if this file can be re-opened + * + * Right now we just try to seek into it, if that fails then that means we + * are probably not dealing with a regular file here and we probably + * cannot reopen it. + */

+ if (lseek(fd, 0, SEEK_SET) == ((off_t) -1)) { + canReopen = false; + }

+ if (virSecurityManagerSetImageFDLabel(driver->securityManager, vm->def, fd) < 0) goto cleanup; if (!(wrapperFd = virFileWrapperFdNew(&fd, path, wrapperFlags))) goto cleanup; + /* Set the header magic */ + if (canReopen) { + /* We will update the magic after the saving completes successfully */ + memcpy(header.magic, QEMU_SAVE_PARTIAL, sizeof(header.magic)); + } else { + /* If we cannot re-open the output, include the final magic here */ + memcpy(header.magic, QEMU_SAVE_MAGIC, sizeof(header.magic));

+ } + /* Write header to file, followed by XML */ if (qemuDomainSaveHeader(fd, path, domXML, &header) < 0) goto cleanup;