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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> | 2012-07-02 14:16:35 +0100 |
| commit | 0437a7905619b8370e25fbae1a3e6388c5277be9 (patch) | |
| tree | 2d06bc5143df6b2c56e418d543e9f0775677d0fb /src/launch.c | |
| parent | 1608ca182b094a1dbe22094f39a5e86f18f728b7 (diff) | |
| download | libguestfs-0437a7905619b8370e25fbae1a3e6388c5277be9.tar.gz libguestfs-0437a7905619b8370e25fbae1a3e6388c5277be9.tar.xz libguestfs-0437a7905619b8370e25fbae1a3e6388c5277be9.zip | |
Add notes about how qemu cache=none works internally.
This is just a comment and has no functional effect.
Diffstat (limited to 'src/launch.c')
| -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 ff451ab2..95694ff8 100644 --- a/src/launch.c +++ b/src/launch.c @@ -297,7 +297,18 @@ 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. */ |