From d082a76d679b019784bc0b131028ee74e381f4a2 Mon Sep 17 00:00:00 2001 From: "Richard W.M. Jones" Date: Wed, 12 Aug 2009 22:12:20 +0100 Subject: [PATCH] add_drive: Don't use cache=off if not supported by underlying filesystem. If you use the guestfs_add_drive function, then currently it generates a qemu command line element like: -drive ...,cache=off,... This causes qemu to try to open the device with O_DIRECT. Unfortunately some filesystems don't support this flag, notably tmpfs, which means you can't use libguestfs in conjunction with tmpfs. On some systems /tmp is a tmpfs filesystem. This patch fixes this so that if the filesystem doesn't support O_DIRECT, then we omit the cache=off parameter. This seems reasonable from a reliability point of view, because if you're using tmpfs then you probably didn't expect reliability in the case where your system suddenly powers off. --- src/generator.ml | 2 ++ src/guestfs.c | 29 +++++++++++++++++++++++------ 2 files changed, 25 insertions(+), 6 deletions(-) diff --git a/src/generator.ml b/src/generator.ml index 3a77202..39faffe 100755 --- a/src/generator.ml +++ b/src/generator.ml @@ -454,6 +454,8 @@ image). This is equivalent to the qemu parameter C<-drive file=filename,cache=off,if=...>. +C is omitted in cases where it is not supported by +the underlying filesystem. Note that this call checks for the existence of C. This stops you from specifying other types of drive which are supported diff --git a/src/guestfs.c b/src/guestfs.c index 37869e8..ad3980f 100644 --- a/src/guestfs.c +++ b/src/guestfs.c @@ -799,14 +799,31 @@ guestfs_add_drive (guestfs_h *g, const char *filename) return -1; } - if (access (filename, F_OK) == -1) { - perrorf (g, "%s", filename); - return -1; + /* cache=off improves reliability in the event of a host crash. + * + * However this option causes qemu to try to open the file with + * O_DIRECT. This fails on some filesystem types (notably tmpfs). + * So we check if we can open the file with or without O_DIRECT, + * and use cache=off (or not) accordingly. + * + * This test also checks for the presence of the file, which + * is a documented semantic of this interface. + */ + int fd = open (filename, O_RDONLY|O_DIRECT); + if (fd >= 0) { + close (fd); + snprintf (buf, len, "file=%s,cache=off,if=" DRIVE_IF, filename); + } else { + fd = open (filename, O_RDONLY); + if (fd >= 0) { + close (fd); + snprintf (buf, len, "file=%s,if=" DRIVE_IF, filename); + } else { + perrorf (g, "%s", filename); + return -1; + } } - /* cache=off improves reliability in the event of a host crash. */ - snprintf (buf, len, "file=%s,cache=off,if=%s", filename, DRIVE_IF); - return guestfs_config (g, "-drive", buf); } -- 1.8.3.1