Re: [Libguestfs] [PATCH nbdkit 2/2] cow: Ignore flush/FUA requests.

There's no point flushing the overlay - a temporary file which has been deleted and is discarded as soon as nbdkit exits. We can gain some performance by avoiding pointlessly flushing the overlay to disk. I also changed can_fua to return NBDKIT_FUA_NATIVE so that NBDKIT_FLAG_FUA is actually being set, rather than nbdkit emulating it with a call to .flush. --- filters/cow/blk.h | 3 --- filters/cow/blk.c | 19 ++----------------- filters/cow/cow.c | 19 +++++++------------ 3 files changed, 9 insertions(+), 32 deletions(-)

+++ b/filters/cow/blk.c @@ -54,9 +54,8 @@ * When writing a block we unconditionally write the data to the * temporary file, setting the bit in the bitmap. * - * We allow the client to request FUA, and emulate it with a flush - * (arguably, since the write overlay is temporary, we could ignore - * FUA altogether). + * Since the overlay is a deleted temporary file, we can ignore FUA + * and flush commands.