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| author | Richard W.M. Jones <rjones@redhat.com> | 2012-07-02 12:47:37 +0100 |
|---|---|---|
| committer | Richard W.M. Jones <rjones@redhat.com> | 2012-07-07 11:24:51 +0100 |
| commit | 3fb00aad0e5209d0f1e027cca5faa68fa4da69d5 (patch) | |
| tree | e9d27c719466f6db04b5b7f994f1ac0f5fc51ce9 | |
| parent | 612b540b3e751959dd3293505d1e9a7569fbf210 (diff) | |
| download | libguestfs-3fb00aad0e5209d0f1e027cca5faa68fa4da69d5.tar.gz libguestfs-3fb00aad0e5209d0f1e027cca5faa68fa4da69d5.tar.xz libguestfs-3fb00aad0e5209d0f1e027cca5faa68fa4da69d5.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 61b903d9..466ec89c 100644 --- a/src/launch.c +++ b/src/launch.c @@ -236,7 +236,18 @@ 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. */ |