[Libguestfs] [nbdkit PATCH] RFC: blocksize: Add test for sharding behavior
Eric Blake
eblake at redhat.com
Thu May 26 02:18:36 UTC 2022
Demonstrate the bug where an aligned write can be lost if it races
between the read and write of a RMW unaligned write.
---
Sending this out for review of the test; it fails (which it is
supposed to as long as I don't fix the blocksize filter), but I hope
it starts passing once I also patch the filter. In the process, I ran
into a libnbd bug where we were using PyByteArray_AsString()
incorrectly, and segfaulting (rather than diagnosing type mismatch) on
something as simple as nbd.Buffer.from_bytearray(b'1'*16) that looks
like it should work; so I posted a patch for that.
tests/Makefile.am | 2 +
tests/test-blocksize-sharding.sh | 161 +++++++++++++++++++++++++++++++
2 files changed, 163 insertions(+)
create mode 100755 tests/test-blocksize-sharding.sh
diff --git a/tests/Makefile.am b/tests/Makefile.am
index b6d30c0a..67e7618b 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1376,11 +1376,13 @@ TESTS += \
test-blocksize.sh \
test-blocksize-extents.sh \
test-blocksize-default.sh \
+ test-blocksize-sharding.sh \
$(NULL)
EXTRA_DIST += \
test-blocksize.sh \
test-blocksize-extents.sh \
test-blocksize-default.sh \
+ test-blocksize-sharding.sh \
$(NULL)
# blocksize-policy filter test.
diff --git a/tests/test-blocksize-sharding.sh b/tests/test-blocksize-sharding.sh
new file mode 100755
index 00000000..09f12051
--- /dev/null
+++ b/tests/test-blocksize-sharding.sh
@@ -0,0 +1,161 @@
+#!/usr/bin/env bash
+# nbdkit
+# Copyright (C) 2018-2022 Red Hat Inc.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#
+# * Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution.
+#
+# * Neither the name of Red Hat nor the names of its contributors may be
+# used to endorse or promote products derived from this software without
+# specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+# THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+# PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+# USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+
+# Demonstrate a fix for a bug where blocksize could lose aligned writes
+# run in parallel with unaligned writes
+
+source ./functions.sh
+set -e
+set -x
+
+requires_plugin eval
+requires_nbdsh_uri
+
+files='blocksize-sharding.img blocksize-sharding.tmp'
+rm -f $files
+cleanup_fn rm -f $files
+
+# Script a server that requires 16-byte aligned requests, and which delays
+# 4s after reads if a witness file exists. Couple it with the delay filter
+# that delays 2s before writes. If an unaligned and aligned write overlap,
+# and can execute in parallel, we would have this timeline:
+#
+# T1 aligned write 1's to 0/16 T2 unaligned write 2's to 4/12
+#t=0 blocksize: next->pwrite(0, 16) blocksize: next->pread(0, 16)
+# delay: wait 2s delay: next->pread(0, 16)
+# ... eval: read 0's, wait 4s
+#t=2 delay: next->pwrite(0, 16) ...
+# eval: write 1's ...
+# return ...
+#t=4 return 0's (now stale)
+# blocksize: next->pwrite(0, 16)
+# delay: wait 2s
+#t=6 delay: next->pwrite(0, 16)
+# eval: write stale RMW buffer
+#
+# leaving us with a sharded 0000222222222222 (T1's write is lost).
+# But as long as the blocksize filter detects the overlap, we should end
+# up with either 1111222222222222 (aligned write completed first), or with
+# 1111111111111111 (unaligned write completed first), either taking 8s,
+# but with no sharding.
+#
+# We also need an nbdsh script that kicks off parallel writes.
+export script='
+import os
+import time
+
+witness = os.getenv("witness")
+
+def touch(path):
+ open(path, "a").close()
+
+# First pass: check that two aligned operations work in parallel
+# Total time should be closer to 2 seconds, rather than 4 if serialized
+print("sanity check")
+ba1 = bytearray(b"1"*16)
+ba2 = bytearray(b"2"*16)
+buf1 = nbd.Buffer.from_bytearray(ba1)
+buf2 = nbd.Buffer.from_bytearray(ba2)
+touch(witness)
+start_t = time.time()
+h.aio_pwrite(buf1, 0)
+h.aio_pwrite(buf2, 0)
+
+while h.aio_in_flight() > 0:
+ h.poll(-1)
+end_t = time.time()
+os.unlink(witness)
+
+# assert h.pread(16, 0) in [b"1"*16, b"2"*16]
+print(h.pread(16,0))
+t = end_t - start_t
+print(t)
+assert t <= 3
+
+# Next pass: try to kick off unaligned first
+print("unaligned first")
+h.zero(16, 0)
+ba3 = bytearray(b"3"*12)
+ba4 = bytearray(b"4"*16)
+buf3 = nbd.Buffer.from_bytearray(ba3)
+buf4 = nbd.Buffer.from_bytearray(ba4)
+touch(witness)
+start_t = time.time()
+h.aio_pwrite(buf3, 4)
+h.aio_pwrite(buf4, 0)
+
+while h.aio_in_flight() > 0:
+ h.poll(-1)
+end_t = time.time()
+os.unlink(witness)
+
+# assert h.pread(16, 0) in [b"4"*4 + b"3"*12, b"4"*16]
+print(h.pread(16,0))
+t = end_t - start_t
+print(t)
+
+# Next pass: try to kick off aligned first
+print("aligned first")
+ba5 = bytearray(b"5"*16)
+ba6 = bytearray(b"6"*12)
+buf5 = nbd.Buffer.from_bytearray(ba5)
+buf6 = nbd.Buffer.from_bytearray(ba6)
+h.zero(16, 0)
+touch(witness)
+start_t = time.time()
+h.aio_pwrite(buf5, 0)
+h.aio_pwrite(buf6, 4)
+
+while h.aio_in_flight() > 0:
+ h.poll(-1)
+end_t = time.time()
+os.unlink(witness)
+
+assert h.pread(16, 0) in [b"5"*4 + b"6"*12, b"5"*16]
+t = end_t - start_t
+print(t)
+'
+
+# Now run everything
+truncate --size=16 blocksize-sharding.img
+export witness="$PWD/blocksize-sharding.tmp"
+nbdkit -U - --filter=blocksize --filter=delay eval delay-write=2 \
+ config='ln -sf "$(realpath "$3")" $tmpdir/$2' \
+ img="$PWD/blocksize-sharding.img" tmp="$PWD/blocksize-sharding.tmp" \
+ get_size='echo 16' block_size='echo 16 64K 1M' \
+ thread_model='echo parallel' \
+ zero='dd if=/dev/zero skip=$4 count=$3 iflag=count_bytes,skip_bytes' \
+ pread='
+ dd if=$tmpdir/img skip=$4 count=$3 iflag=count_bytes,skip_bytes
+ if [ -f $tmpdir/tmp ]; then sleep 4; fi ' \
+ pwrite='dd of=$tmpdir/img seek=$4 conv=notrunc oflag=seek_bytes' \
+ --run 'nbdsh -u "$uri" -c "$script"'
--
2.36.1
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