Re: [Libvir] Reopening the old discussion about virDomainBlockPeek

Virt-df[1] has now gained the ability to fully parse LVM2 partitions, thus: # virt-df -c qemu:///system -h Filesystem Size Used Available Type rhel51x32kvm:hda1 96.8 MiB 14.6 MiB 82.2 MiB Linux ext2/3 rhel51x32kvm:VolGroup00/LogVol00 6.4 GiB 3.6 GiB 2.8 GiB Linux ext2/3 rhel51x32kvm:VolGroup00/LogVol01 992.0 MiB Linux swap However it still has to do it by opening the local partitions / files, which means it isn't using a "proper" part of libvirt and more importantly it cannot run remotely. I'd like to find out whether the time has come for us to look again at a virDomainBlockPeek call for libvirt. Here is the original thread plus patch from 7 months ago: http://www.redhat.com/archives/libvir-list/2007-October/thread.html#00089 (I've attached an updated patch against current CVS). I appreciate that some cases might not be simple (iSCSI?), but the same argument applies to block device statistics too, and we make those available where possible. I think a best-effort call allowing callers to peek into the block devices of guests would be very useful.

Index: configure.in =================================================================== RCS file: /data/cvs/libvirt/configure.in,v retrieving revision 1.139 diff -u -r1.139 configure.in --- configure.in 8 Apr 2008 16:45:57 -0000 1.139 +++ configure.in 16 Apr 2008 15:18:07 -0000 @@ -60,6 +60,10 @@ LIBVIRT_COMPILE_WARNINGS(maximum) +dnl Support large files / 64 bit seek offsets. +dnl Use --disable-largefile if you don't want this. +AC_SYS_LARGEFILE +

- +int virDomainBlockPeek (virDomainPtr dom, + const char *path, + long long offset, + size_t size, + void *buffer);

+/* "domblkpeek" command + */ +static vshCmdInfo info_domblkpeek[] = { + {"syntax", "domblkpeek <domain> <path> <offset> <size>"}, + {"help", gettext_noop("peek at a domain block device")}, + {"desc", gettext_noop("Peek into a domain block device.")}, + {NULL,NULL} +}; + +static vshCmdOptDef opts_domblkpeek[] = { + {"domain", VSH_OT_DATA, VSH_OFLAG_REQ, gettext_noop("domain name, id or uuid")}, + {"path", VSH_OT_DATA, VSH_OFLAG_REQ, gettext_noop("block device path")}, + {"offset", VSH_OT_DATA, VSH_OFLAG_REQ, gettext_noop("start offset")}, + {"size", VSH_OT_DATA, VSH_OFLAG_REQ, gettext_noop("size in bytes")}, + {NULL, 0, 0, NULL} +};

+ /* The path is correct, now try to open it and get its size. */ + fd = open (path, O_RDONLY); + if (fd == -1 || fstat (fd, &statbuf) == -1) { + virXendError (domain->conn, VIR_ERR_SYSTEM_ERROR, strerror (errno)); + goto done; + } + + /* XXX The following test fails for LVM devices for a couple + * of reasons: (1) They are commonly symlinks to /dev/mapper/foo + * and despite the man page for fstat, fstat stats the link not + * the file. (2) Stat even on the block device returns st_size==0. + * + * Anyhow, it should be safe to ignore this test since we are + * in O_RDONLY mode. + */ +#if 0 + /* NB we know offset > 0, size >= 0 */ + if (offset + size > statbuf.st_size) { + virXendError (domain->conn, VIR_ERR_INVALID_ARG, "offset"); + goto done; + } +#endif

+ + /* Seek and read. */ + /* NB. Because we configure with AC_SYS_LARGEFILE, off_t should + * be 64 bits on all platforms. + */ + if (lseek (fd, offset, SEEK_SET) == (off_t) -1 || + saferead (fd, buffer, size) == (ssize_t) -1) { + virXendError (domain->conn, VIR_ERR_SYSTEM_ERROR, strerror (errno)); + goto done; + } + + ret = 0; + done: + if (fd >= 0) close (fd); + return ret; +} + #endif /* ! PROXY */ #endif /* WITH_XEN */ Index: src/xend_internal.h =================================================================== RCS file: /data/cvs/libvirt/src/xend_internal.h,v retrieving revision 1.40 diff -u -r1.40 xend_internal.h --- src/xend_internal.h 10 Apr 2008 16:54:54 -0000 1.40 +++ src/xend_internal.h 16 Apr 2008 15:18:26 -0000 @@ -228,6 +228,8 @@ int xenDaemonDomainMigratePrepare (virConnectPtr dconn, char **cookie, int *cookielen, const char *uri_in, char **uri_out, unsigned long flags, const char *dname, unsigned long resource); int xenDaemonDomainMigratePerform (virDomainPtr domain, const char *cookie, int cookielen, const char *uri, unsigned long flags, const char *dname, unsigned long resource); +int xenDaemonDomainBlockPeek (virDomainPtr domain, const char *path, long long offset, size_t size, void *buffer); + #ifdef __cplusplus } #endif Index: src/xm_internal.c =================================================================== RCS file: /data/cvs/libvirt/src/xm_internal.c,v retrieving revision 1.70 diff -u -r1.70 xm_internal.c --- src/xm_internal.c 10 Apr 2008 16:54:54 -0000 1.70 +++ src/xm_internal.c 16 Apr 2008 15:18:29 -0000 @@ -3160,4 +3160,15 @@ return (ret); } +int +xenXMDomainBlockPeek (virDomainPtr dom, + const char *path ATTRIBUTE_UNUSED, + long long offset ATTRIBUTE_UNUSED, + size_t size ATTRIBUTE_UNUSED, + void *buffer ATTRIBUTE_UNUSED) +{ + xenXMError (dom->conn, VIR_ERR_NO_SUPPORT, __FUNCTION__); + return -1; +}