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Re: [Libguestfs] [PATCH] Revert "add_drive_ro adds readonly=on option if available." (RHBZ#617200).



On 22/07/10 14:46, Richard W.M. Jones wrote:
From 004c0345d6fd9f141978f5163e053a67dba31cd0 Mon Sep 17 00:00:00 2001
From: Richard Jones<rjones redhat com>
Date: Thu, 22 Jul 2010 14:39:36 +0100
Subject: [PATCH] Revert "add_drive_ro adds readonly=on option if available." (RHBZ#617200).

Adding the readonly=on option is not so clever.  This causes
qemu to present the disk as read-only to the guest.  (The
expected behaviour of snapshots=on,readonly=on was that it
would open the disk O_RDONLY but present a writable disk to
the guest).

Since the guest sees a read-only disk, we are unable to do any
recovery if a filesystem on the disk is inconsistent.  This basically
prevents most accesses to live disk images.

What we really want is a qemu option which presents a writable
disk to the guest, but only opens the disk on the host side with
O_RDONLY, to alleviate the udev bug RHBZ#571714.

This reverts commit 676462684e05dd8341dd695762dd99a87d8ec022.
---
  src/generator.ml |    4 +---
  src/guestfs.c    |   22 ++++------------------
  2 files changed, 5 insertions(+), 21 deletions(-)


...

diff --git a/src/guestfs.c b/src/guestfs.c
index 85a042a..d6c8d60 100644
--- a/src/guestfs.c
+++ b/src/guestfs.c
@@ -836,6 +836,9 @@ int
  guestfs__add_drive_ro_with_if (guestfs_h *g, const char *filename,
                                 const char *drive_if)
  {
+  size_t len = strlen (filename) + 64;
+  char buf[len];
+
    if (strchr (filename, ',') != NULL) {
      error (g, _("filename cannot contain ',' (comma) character"));
      return -1;

Could you please move these 2 lines...

@@ -846,24 +849,7 @@ guestfs__add_drive_ro_with_if (guestfs_h *g, const char *filename,
      return -1;
    }

-  if (qemu_supports (g, NULL) == -1)
-    return -1;
-
-  /* Only SCSI and virtio drivers support readonly mode.
-   * This is only supported as a QEMU feature since 2010/01.
-   */
-  int supports_ro = 0;
-  if ((STREQ (drive_if, "scsi") || STREQ (drive_if, "virtio"))&&
-      qemu_supports (g, "readonly=on"))
-    supports_ro = 1;
-
-  size_t len = strlen (filename) + 100;
-  char buf[len];
-
-  snprintf (buf, len, "file=%s,snapshot=on,%sif=%s",
-            filename,
-            supports_ro ? "readonly=on," : "",
-            drive_if);

... down here?

+  snprintf (buf, len, "file=%s,snapshot=on,if=%s", filename, drive_if);

    return guestfs__config (g, "-drive", buf);
  }

Apart from that, ACK. FWIW, I've also cogitated over the qemu man page definition of what snapshot does, and as long as we can never send "Ctrl-a s" to the qemu process this seems to be safe.

Matt
--
Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team

GPG ID:  D33C3490
GPG FPR: 3733 612D 2D05 5458 8A8A 1600 3441 EA19 D33C 3490


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