[libvirt] Re: [PATCH] also allow use of XZ for Qemu image compression

[Jim Meyering]

Jim Meyering wrote:
> Chris Lalancette wrote:
>> Jim Meyering wrote:
>>> While this patch stays minimal by simply adding XZ/xz to the list,
>>> I think it would be better to remove lzma, since it uses
>>> an inferior format (which lacks an integrity check), and
>>> has been effectively subsumed by xz.
>>>
>>> Let me know if you'd like that, and I'll prepare the slightly
>>> more invasive patch.
>>
>> I'm on the fence about it.  While I do understand the situation now (thanks for
>> explaining), I think keeping lzma for compatibility with older distros might be
>> a good idea.  Either way, we have to keep the LZMA slot in the enum "free",
>> since it's part of the on-disk ABI for the save format.  And on that note...
>>
>>> diff --git a/src/qemu_driver.c b/src/qemu_driver.c
>>> index f64d70b..7b64712 100644
>>> --- a/src/qemu_driver.c
>>> +++ b/src/qemu_driver.c
>>> @@ -3622,7 +3622,8 @@ enum qemud_save_formats {
>>>      QEMUD_SAVE_FORMAT_RAW,
>>>      QEMUD_SAVE_FORMAT_GZIP,
>>>      QEMUD_SAVE_FORMAT_BZIP2,
>>> -    QEMUD_SAVE_FORMAT_LZMA,
>>> +    QEMUD_SAVE_FORMAT_LZMA,  /* deprecated, in favor of xz */
>>> +    QEMUD_SAVE_FORMAT_XZ,
>>>      QEMUD_SAVE_FORMAT_LZOP,
>>>  };
>>
>> You'll need to add QEMUD_SAVE_FORMAT_XZ to the end of the enum, to maintain
>> on-disk compatibility.  Otherwise it looks good.
>
> Thanks.  Good point.  Here's an amended patch:
> Note, I've reordered the if/else placement to match enum member
> ordering, too.
...

At Chris' suggestion, I've added a comment warning not to do what I did ;-)

To make it even more obvious that these numbers matter,
I've assigned explicit constants in the enum:

  +    QEMUD_SAVE_FORMAT_RAW = 0,
  +    QEMUD_SAVE_FORMAT_GZIP = 1,
  +    QEMUD_SAVE_FORMAT_BZIP2 = 2,

Currently, if someone accidentally adds this (mistakenly reused number):

  +    QEMUD_SAVE_FORMAT_OOPS = 2,

the compiler won't catch it.
However, with a small additional change to use VIR_ENUM_DECL
and VIR_ENUM_IMPL, thus replacing the if/elsif/ chains,
and the compiler *would* detect that mistake.
It'd also help to factor out the QEMUD_SAVE_FORMAT_<> to "<>"
association, rather than having literals like "gzip" and "bzip2"
in two places.

Now that I've described that, I'll prepare a separate patch.
For now, here's the added comment and enum initializers.


>From ef653538806f981ca6ca5bb270e7a8fb1618f6d4 Mon Sep 17 00:00:00 2001
From: Jim Meyering <meyering at redhat.com>
Date: Tue, 8 Sep 2009 20:52:37 +0200
Subject: [PATCH] also allow use of XZ for Qemu image compression

* src/qemu_driver.c (enum qemud_save_formats) [QEMUD_SAVE_FORMAT_XZ]:
New member.
[QEMUD_SAVE_FORMAT_LZMA]: Mark as deprecated.
Use an explicit value for each member.
(qemudDomainSave, qemudDomainRestore): Handle the new member.
* src/qemu.conf: Mention xz, too.
---
 src/qemu.conf     |    2 +-
 src/qemu_driver.c |   21 ++++++++++++++++-----
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/src/qemu.conf b/src/qemu.conf
index 06babc4..342bb8a 100644
--- a/src/qemu.conf
+++ b/src/qemu.conf
@@ -134,7 +134,7 @@
 # memory from the domain is dumped out directly to a file.  If you have
 # guests with a large amount of memory, however, this can take up quite
 # a bit of space.  If you would like to compress the images while they
-# are being saved to disk, you can also set "gzip", "bzip2", "lzma"
+# are being saved to disk, you can also set "gzip", "bzip2", "lzma", "xz",
 # or "lzop" for save_image_format.  Note that this means you slow down
 # the process of saving a domain in order to save disk space.
 #
diff --git a/src/qemu_driver.c b/src/qemu_driver.c
index f64d70b..0cdcd98 100644
--- a/src/qemu_driver.c
+++ b/src/qemu_driver.c
@@ -3619,11 +3619,15 @@ static char *qemudEscapeShellArg(const char *in)
 #define QEMUD_SAVE_VERSION 2

 enum qemud_save_formats {
-    QEMUD_SAVE_FORMAT_RAW,
-    QEMUD_SAVE_FORMAT_GZIP,
-    QEMUD_SAVE_FORMAT_BZIP2,
-    QEMUD_SAVE_FORMAT_LZMA,
-    QEMUD_SAVE_FORMAT_LZOP,
+    QEMUD_SAVE_FORMAT_RAW = 0,
+    QEMUD_SAVE_FORMAT_GZIP = 1,
+    QEMUD_SAVE_FORMAT_BZIP2 = 2,
+    QEMUD_SAVE_FORMAT_LZMA = 3,  /* deprecated, in favor of xz */
+    QEMUD_SAVE_FORMAT_LZOP = 4,
+    QEMUD_SAVE_FORMAT_XZ = 5,
+    /* Note: add new members only at the end.
+       These values are used in the on-disk format.
+       Do not change or re-use numbers. */
 };

 struct qemud_save_header {
@@ -3666,6 +3670,8 @@ static int qemudDomainSave(virDomainPtr dom,
         header.compressed = QEMUD_SAVE_FORMAT_LZMA;
     else if (STREQ(driver->saveImageFormat, "lzop"))
         header.compressed = QEMUD_SAVE_FORMAT_LZOP;
+    else if (STREQ(driver->saveImageFormat, "xz"))
+        header.compressed = QEMUD_SAVE_FORMAT_XZ;
     else {
         qemudReportError(dom->conn, dom, NULL, VIR_ERR_OPERATION_FAILED,
                          "%s", _("Invalid save image format specified in configuration file"));
@@ -3761,6 +3767,9 @@ static int qemudDomainSave(virDomainPtr dom,
     else if (header.compressed == QEMUD_SAVE_FORMAT_LZOP)
         internalret = virAsprintf(&command, "migrate \"exec:"
                                   "lzop -c >> '%s' 2>/dev/null\"", safe_path);
+    else if (header.compressed == QEMUD_SAVE_FORMAT_XZ)
+        internalret = virAsprintf(&command, "migrate \"exec:"
+                                  "xz -c >> '%s' 2>/dev/null\"", safe_path);
     else {
         qemudReportError(dom->conn, dom, NULL, VIR_ERR_INTERNAL_ERROR,
                          _("Invalid compress format %d"),
@@ -4385,6 +4394,8 @@ static int qemudDomainRestore(virConnectPtr conn,
             intermediate_argv[0] = "lzma";
         else if (header.compressed == QEMUD_SAVE_FORMAT_LZOP)
             intermediate_argv[0] = "lzop";
+        else if (header.compressed == QEMUD_SAVE_FORMAT_XZ)
+            intermediate_argv[0] = "xz";
         else if (header.compressed != QEMUD_SAVE_FORMAT_RAW) {
             qemudReportError(conn, NULL, NULL, VIR_ERR_OPERATION_FAILED,
                              _("Unknown compressed save format %d"),
--
1.6.5.rc0.164.g5f6b0