[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(-) diff --git a/filters/cow/blk.h b/filters/cow/blk.h index a50b6ca3..28ef3b4a 100644 --- a/filters/cow/blk.h +++ b/filters/cow/blk.h @@ -77,7 +77,4 @@ extern int blk_cache (struct nbdkit_next_ops *next_ops, void *nxdata, extern int blk_write (uint64_t blknum, const uint8_t *block, int *err) __attribute__((__nonnull__ (2, 3))); -/* Flush the overlay to disk. */ -extern int blk_flush (void); - #endif /* NBDKIT_BLK_H */ diff --git a/filters/cow/blk.c b/filters/cow/blk.c index 10af4a84..9e85920f 100644 --- a/filters/cow/blk.c +++ 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. */ #include <config.h> @@ -260,17 +259,3 @@ blk_write (uint64_t blknum, const uint8_t *block, int *err) return 0; } - -int -blk_flush (void) -{ - /* I think we don't care about file metadata for this temporary - * file, so only flush the data. - */ - if (fdatasync (fd) == -1) { - nbdkit_error ("fdatasync: %m"); - return -1; - } - - return 0; -} diff --git a/filters/cow/cow.c b/filters/cow/cow.c index cbb072f1..92358375 100644 --- a/filters/cow/cow.c +++ b/filters/cow/cow.c @@ -171,7 +171,7 @@ cow_can_flush (struct nbdkit_next_ops *next_ops, void *nxdata, void *handle) static int cow_can_fua (struct nbdkit_next_ops *next_ops, void *nxdata, void *handle) { - return NBDKIT_FUA_EMULATE; + return NBDKIT_FUA_NATIVE; } static int @@ -343,8 +343,8 @@ cow_pwrite (struct nbdkit_next_ops *next_ops, void *nxdata, return -1; } - if (flags & NBDKIT_FLAG_FUA) - return cow_flush (next_ops, nxdata, handle, 0, err); + /* flags & NBDKIT_FLAG_FUA is deliberately ignored. */ + return 0; } @@ -426,8 +426,8 @@ cow_zero (struct nbdkit_next_ops *next_ops, void *nxdata, return -1; } - if (flags & NBDKIT_FLAG_FUA) - return cow_flush (next_ops, nxdata, handle, 0, err); + /* flags & NBDKIT_FLAG_FUA is deliberately ignored. */ + return 0; } @@ -435,13 +435,8 @@ static int cow_flush (struct nbdkit_next_ops *next_ops, void *nxdata, void *handle, uint32_t flags, int *err) { - int r; - - ACQUIRE_LOCK_FOR_CURRENT_SCOPE (&lock); - r = blk_flush (); - if (r == -1) - *err = errno; - return r; + /* Deliberately ignored. */ + return 0; } static int -- 2.29.0.rc2