bug#47225: QEMU warning about performance

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

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Hi Leo,

Leo Famulari <leo@famulari.name> writes:

> While using `guix system vm`, I noticed this warning. I think it's new,
> maybe from the 5.2.0 update?
>
> ------
> qemu-system-x86_64: warning: 9p: degraded performance: a reasonable
> high msize should be chosen on client/guest side (chosen msize is <=
> 8192). See https://wiki.qemu.org/Documentation/9psetup#msize for
> details.
> ------
>
> That web page suggests that we increase the virtio packet size when
> declaring the 9p file-systems, I think in %linux-vm-file-systems of (gnu
> system vm).

The attached patch should fix it (it does for me at least!).


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From c720e68229322e5c38c0321b021e8d6430636111 Mon Sep 17 00:00:00 2001
From: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Date: Tue, 6 Apr 2021 17:37:33 -0400
Subject: [PATCH] system: vm: Set a larger value for the msize option of the 9p
 file system.

Fixes <https://issues.guix.gnu.org/47225>.

* gnu/system/vm.scm (mapping->file-system): Set the msize option to a value of
100 MiB (from the default 8 KiB).

Reported-by: Leo Famulari <leo@famulari.name>
---
 gnu/system/vm.scm | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/gnu/system/vm.scm b/gnu/system/vm.scm
index 3d0935b3af..f9cf39ae42 100644
--- a/gnu/system/vm.scm
+++ b/gnu/system/vm.scm
@@ -581,7 +581,15 @@ the operating system."
        (type "9p")
        (flags (if writable? '() '(read-only)))
        (options (string-append "trans=virtio"
-                               (if writable? "" ",cache=loose")))
+                               (if writable? "" ",cache=loose")
+                               ;; By default, the msize value is 8 KiB, which
+                               ;; according to QEMU is insufficient and would
+                               ;; degrade performance.  The msize value should
+                               ;; roughly match the bandwidth of the system's
+                               ;; IO (see:
+                               ;; https://wiki.qemu.org/Documentation/9psetup#msize).
+                               ;; Use 100 MiB as a conservative default.
+                               ",msize=104857600"))
        (check? #f)
        (create-mount-point? #t)))))
 
-- 
2.31.1


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Thank you for the report,

Maxim