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author | Richard W.M. Jones <rjones@redhat.com> | 2012-07-02 12:47:37 +0100 |
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committer | Richard W.M. Jones <rjones@redhat.com> | 2012-07-06 17:36:30 +0100 |
commit | 7876f604e9b58a73d7394ed373795ad7707293a4 (patch) | |
tree | 53e9a4e97ca72f699d065a9080e12b61d47f8573 | |
parent | 25f20c98bf000a46c424f792e04fd04c7b17bcad (diff) | |
download | libguestfs-7876f604e9b58a73d7394ed373795ad7707293a4.tar.gz libguestfs-7876f604e9b58a73d7394ed373795ad7707293a4.tar.xz libguestfs-7876f604e9b58a73d7394ed373795ad7707293a4.zip |
Add notes about how qemu cache=none works internally.
This is just a comment and has no functional effect.
(cherry picked from commit 0437a7905619b8370e25fbae1a3e6388c5277be9)
-rw-r--r-- | src/launch.c | 13 |
1 files changed, 12 insertions, 1 deletions
diff --git a/src/launch.c b/src/launch.c index 66b77d2a..abfd1d05 100644 --- a/src/launch.c +++ b/src/launch.c @@ -294,7 +294,18 @@ guestfs__config (guestfs_h *g, * So we check if we can open the file with or without O_DIRECT, * and use cache=none (or not) accordingly. * - * NB: This function is only called on the !readonly path. We must + * Notes: + * + * (1) In qemu, cache=none and cache=off are identical. + * + * (2) cache=none does not disable caching entirely. qemu still + * maintains a writeback cache internally, which will be written out + * when qemu is killed (with SIGTERM). It disables *host kernel* + * caching by using O_DIRECT. To disable caching entirely in kernel + * and qemu we would need to use cache=directsync but there is a + * performance penalty for that. + * + * (3) This function is only called on the !readonly path. We must * try to open with O_RDWR to test that the file is readable and * writable here. */ |