flamegraphs: Move demo of flamegraph earlier

[libguestfs-talks.git] / 2019-kvm-forum / notes-04-fast-zero

diff --git a/2019-kvm-forum/notes-04-fast-zero b/2019-kvm-forum/notes-04-fast-zero
index 19622d0..c818417 100644 (file)
--- a/2019-kvm-forum/notes-04-fast-zero
+++ b/2019-kvm-forum/notes-04-fast-zero
@@ -53,15 +53,14 @@ didn't really help, for a couple of reasons.  If writing zeroes is
 fast, checking the destination first is either a mere tradeoff in
 commands (BLOCK_STATUS replacing WRITE_ZEROES when the destination is
 already zero) or a pessimization (BLOCK_STATUS still has to be
-followed by WRITE_ZEROES).  And if writing zeroes is slow, we have a
-speedup holes when BLOCK_STATUS itself is fast on pre-existing
-destination holes, but we encountered situations such as tmpfs that
-has a linear rather than constant-time lseek(SEEK_HOLE)
-implementation, where we ended up with quadratic behavior all due to
-BLOCK_STATUS calls.  Thus, for now, qemu-img convert does not use
-BLOCK_STATUS, and as mentioned earlier, I used the noextents filter in
-my test case to ensure BLOCK_STATUS is not interfering with timing
-results.
+followed by WRITE_ZEROES).  Conversely, if writing zeroes is slow, the
+penalty of the extra check when the destination is not a hole could be
+in the noise, but whether or not we have a definite speedup for
+avoiding WRITE_ZEROES depends on whether BLOCK_STATUS itself is fast -
+yet we encountered situations such as tmpfs that has a linear rather
+than constant-time lseek(SEEK_HOLE) implementation, where we ended up
+with quadratic behavior all due to BLOCK_STATUS calls.  Thus, for now,
+qemu-img convert does not use BLOCK_STATUS.
 
 * Heading: Pre-zeroing: a tale of two servers
 - 4400- .term