Log Spacemap Project

= Motivation

At Delphix we've seen a lot of customer systems where fragmentation
is over 75% and random writes take a performance hit because a lot
of time is spend on I/Os that update on-disk space accounting metadata.
Specifically, we seen cases where 20% to 40% of sync time is spend
after sync pass 1 and ~30% of the I/Os on the system is spent updating
spacemaps.

The problem is that these pools have existed long enough that we've
touched almost every metaslab at least once, and random writes
scatter frees across all metaslabs every TXG, thus appending to
their spacemaps and resulting in many I/Os. To give an example,
assuming that every VDEV has 200 metaslabs and our writes fit within
a single spacemap block (generally 4K) we have 200 I/Os. Then if we
assume 2 levels of indirection, we need 400 additional I/Os and
since we are talking about metadata for which we keep 2 extra copies
for redundancy we need to triple that number, leading to a total of
1800 I/Os per VDEV every TXG.

We could try and decrease the number of metaslabs so we have less
I/Os per TXG but then each metaslab would cover a wider range on
disk and thus would take more time to be loaded in memory from disk.
In addition, after it's loaded, it's range tree would consume more
memory.

Another idea would be to just increase the spacemap block size
which would allow us to fit more entries within an I/O block
resulting in fewer I/Os per metaslab and a speedup in loading time.
The problem is still that we don't deal with the number of I/Os
going up as the number of metaslabs is increasing and the fact
is that we generally write a lot to a few metaslabs and a little
to the rest of them. Thus, just increasing the block size would
actually waste bandwidth because we won't be utilizing our bigger
block size.

= About this patch

This patch introduces the Log Spacemap project which provides the
solution to the above problem while taking into account all the
aforementioned tradeoffs. The details on how it achieves that can
be found in the references sections below and in the code (see
Big Theory Statement in spa_log_spacemap.c).

Even though the change is fairly constraint within the metaslab
and lower-level SPA codepaths, there is a side-change that is
user-facing. The change is that VDEV IDs from VDEV holes will no
longer be reused. To give some background and reasoning for this,
when a log device is removed and its VDEV structure was replaced
with a hole (or was compacted; if at the end of the vdev array),
its vdev_id could be reused by devices added after that. Now
with the pool-wide space maps recording the vdev ID, this behavior
can cause problems (e.g. is this entry referring to a segment in
the new vdev or the removed log?). Thus, to simplify things the
ID reuse behavior is gone and now vdev IDs for top-level vdevs
are truly unique within a pool.

= Testing

The illumos implementation of this feature has been used internally
for a year and has been in production for ~6 months. For this patch
specifically there don't seem to be any regressions introduced to
ZTS and I have been running zloop for a week without any related
problems.

= Performance Analysis (Linux Specific)

All performance results and analysis for illumos can be found in
the links of the references. Redoing the same experiments in Linux
gave similar results. Below are the specifics of the Linux run.

After the pool reached stable state the percentage of the time
spent in pass 1 per TXG was 64% on average for the stock bits
while the log spacemap bits stayed at 95% during the experiment
(graph: sdimitro.github.io/img/linux-lsm/PercOfSyncInPassOne.png).

Sync times per TXG were 37.6 seconds on average for the stock
bits and 22.7 seconds for the log spacemap bits (related graph:
sdimitro.github.io/img/linux-lsm/SyncTimePerTXG.png). As a result
the log spacemap bits were able to push more TXGs, which is also
the reason why all graphs quantified per TXG have more entries for
the log spacemap bits.

Another interesting aspect in terms of txg syncs is that the stock
bits had 22% of their TXGs reach sync pass 7, 55% reach sync pass 8,
and 20% reach 9. The log space map bits reached sync pass 4 in 79%
of their TXGs, sync pass 7 in 19%, and sync pass 8 at 1%. This
emphasizes the fact that not only we spend less time on metadata
but we also iterate less times to convergence in spa_sync() dirtying
objects.
[related graphs:
stock- sdimitro.github.io/img/linux-lsm/NumberOfPassesPerTXGStock.png
lsm- sdimitro.github.io/img/linux-lsm/NumberOfPassesPerTXGLSM.png]

Finally, the improvement in IOPs that the userland gains from the
change is approximately 40%. There is a consistent win in IOPS as
you can see from the graphs below but the absolute amount of
improvement that the log spacemap gives varies within each minute
interval.
sdimitro.github.io/img/linux-lsm/StockVsLog3Days.png
sdimitro.github.io/img/linux-lsm/StockVsLog10Hours.png

= Porting to Other Platforms

For people that want to port this commit to other platforms below
is a list of ZoL commits that this patch depends on:

Make zdb results for checkpoint tests consistent
db587941c5

Update vdev_is_spacemap_addressable() for new spacemap encoding
419ba59145

Simplify spa_sync by breaking it up to smaller functions
8dc2197b7b

Factor metaslab_load_wait() in metaslab_load()
b194fab0fb

Rename range_tree_verify to range_tree_verify_not_present
df72b8bebe

Change target size of metaslabs from 256GB to 16GB
c853f382db

zdb -L should skip leak detection altogether
21e7cf5da8

vs_alloc can underflow in L2ARC vdevs
7558997d2f

Simplify log vdev removal code
6c926f426a

Get rid of space_map_update() for ms_synced_length
425d3237ee

Introduce auxiliary metaslab histograms
928e8ad47d

Error path in metaslab_load_impl() forgets to drop ms_sync_lock
8eef997679

= References

Background, Motivation, and Internals of the Feature
- OpenZFS 2017 Presentation:
youtu.be/jj2IxRkl5bQ
- Slides:
slideshare.net/SerapheimNikolaosDim/zfs-log-spacemaps-project

Flushing Algorithm Internals & Performance Results
(Illumos Specific)
- Blogpost:
sdimitro.github.io/post/zfs-lsm-flushing/
- OpenZFS 2018 Presentation:
youtu.be/x6D2dHRjkxw
- Slides:
slideshare.net/SerapheimNikolaosDim/zfs-log-spacemap-flushing-algorithm

Upstream Delphix Issues:
DLPX-51539, DLPX-59659, DLPX-57783, DLPX-61438, DLPX-41227, DLPX-59320
DLPX-63385

Reviewed-by: Sean Eric Fagan <sef@ixsystems.com>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #8442

tests/runfiles/linux.run

@@ -927,3 +927,9 @@ tags = ['functional', 'zvol', 'zvol_swap']
 [tests/functional/libzfs]
 tests = ['many_fds', 'libzfs_input']
 tags = ['functional', 'libzfs']
+
+[tests/functional/log_spacemap]
+tests = ['log_spacemap_import_logs']
+pre =
+post =
+tags = ['functional', 'log_spacemap']

tests/zfs-tests/tests/functional/Makefile.am

@@ -33,8 +33,8 @@ SUBDIRS = \
 	largest_pool \
 	libzfs \
 	limits \
-	pyzfs \
 	link_count \
+	log_spacemap \
 	migration \
 	mmap \
 	mmp \
@@ -50,6 +50,7 @@ SUBDIRS = \
 	privilege \
 	procfs \
 	projectquota \
+	pyzfs \
 	quota \
 	raidz \
 	redacted_send \

tests/zfs-tests/tests/functional/arc/dbufstats_001_pos.ksh

@@ -58,7 +58,7 @@ function testdbufstat # stat_name dbufstat_filter
 	from_dbufstat=$(grep -w "$name" "$DBUFSTATS_FILE" | awk '{ print $3 }')
 	from_dbufs=$(dbufstat -bxn -i "$DBUFS_FILE" "$filter" | wc -l)
 
-	within_tolerance $from_dbufstat $from_dbufs 9 \
+	within_tolerance $from_dbufstat $from_dbufs 15 \
 	    || log_fail "Stat $name exceeded tolerance"
 }
 

tests/zfs-tests/tests/functional/cli_root/zpool_get/zpool_get.cfg

@@ -79,6 +79,7 @@ typeset -a properties=(
     "feature@redaction_bookmarks"
     "feature@redacted_datasets"
     "feature@bookmark_written"
+    "feature@log_spacemap"
 )
 
 # Additional properties added for Linux.

tests/zfs-tests/tests/functional/log_spacemap/Makefile.am

@@ -0,0 +1,2 @@
+pkgdatadir = $(datadir)/@PACKAGE@/zfs-tests/tests/functional/log_spacemap
+dist_pkgdata_SCRIPTS = log_spacemap_import_logs.ksh

tests/zfs-tests/tests/functional/log_spacemap/log_spacemap_import_logs.ksh

@@ -0,0 +1,81 @@
+#! /bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2019 by Delphix. All rights reserved.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+#
+# DESCRIPTION:
+# Log spacemaps are generally destroyed at export in order to
+# not induce performance overheads at import time. As a result,
+# the log spacemap codepaths that read the logs in import times
+# are not tested outside of ztest and pools with DEBUG bits doing
+# many imports/exports while running the test suite.
+#
+# This test uses an internal tunable and forces ZFS to keep the
+# log spacemaps at export, and then re-imports the pool, thus
+# providing explicit testing of those codepaths. It also uses
+# another tunable to load all the metaslabs when the pool is
+# re-imported so more assertions and verifications will be hit.
+#
+# STRATEGY:
+#	1. Create pool.
+#	2. Do a couple of writes to generate some data for spacemap logs.
+#	3. Set tunable to keep logs after export.
+#	4. Export pool and verify that there are logs with zdb.
+#	5. Set tunable to load all metaslabs at import.
+#	6. Import pool.
+#	7. Reset tunables.
+#
+
+verify_runnable "global"
+
+function cleanup
+{
+	log_must set_tunable64 zfs_keep_log_spacemaps_at_export 0
+	log_must set_tunable64 metaslab_debug_load 0
+	if poolexists $LOGSM_POOL; then
+		log_must zpool destroy -f $LOGSM_POOL
+	fi
+}
+log_onexit cleanup
+
+LOGSM_POOL="logsm_import"
+TESTDISK="$(echo $DISKS | cut -d' ' -f1)"
+
+log_must zpool create -o cachefile=none -f $LOGSM_POOL $TESTDISK
+log_must zfs create $LOGSM_POOL/fs
+
+log_must dd if=/dev/urandom of=/$LOGSM_POOL/fs/00 bs=128k count=10
+log_must sync
+log_must dd if=/dev/urandom of=/$LOGSM_POOL/fs/00 bs=128k count=10
+log_must sync
+
+log_must set_tunable64 zfs_keep_log_spacemaps_at_export 1
+log_must zpool export $LOGSM_POOL
+
+LOGSM_COUNT=$(zdb -m -e $LOGSM_POOL | grep "Log Spacemap object" | wc -l)
+if (( LOGSM_COUNT == 0 )); then
+	log_fail "Pool does not have any log spacemaps after being exported"
+fi
+
+log_must set_tunable64 metaslab_debug_load 1
+log_must zpool import $LOGSM_POOL
+
+log_pass "Log spacemaps imported with no errors"

tests/zfs-tests/tests/functional/removal/removal_condense_export.ksh

@@ -30,7 +30,7 @@ function reset
 
 default_setup_noexit "$DISKS" "true"
 log_onexit reset
-log_must set_tunable64 zfs_condense_indirect_commit_entry_delay_ms 1000
+log_must set_tunable64 zfs_condense_indirect_commit_entry_delay_ms 5000
 log_must set_tunable64 zfs_condense_min_mapping_bytes 1
 
 log_must zfs set recordsize=512 $TESTPOOL/$TESTFS
@@ -82,7 +82,7 @@ log_mustnot vdevs_in_pool $TESTPOOL $REMOVEDISK
 log_must stride_dd -i /dev/urandom -o $TESTDIR/file -b 512 -c 20 -s 1024
 
 sync_pool $TESTPOOL
-sleep 5
+sleep 4
 sync_pool $TESTPOOL
 log_must zpool export $TESTPOOL
 zdb -e -p $REMOVEDISKPATH $TESTPOOL | grep 'Condensing indirect vdev' || \