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mirror_zfs/include/sys/metaslab.h
Serapheim Dimitropoulos 93e28d661e 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
2019-07-16 10:11:49 -07:00

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/*
* CDDL HEADER START
*
* The contents of this file are subject to the terms of the
* Common Development and Distribution License (the "License").
* You may not use this file except in compliance with the License.
*
* You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
* or http://www.opensolaris.org/os/licensing.
* See the License for the specific language governing permissions
* and limitations under the License.
*
* When distributing Covered Code, include this CDDL HEADER in each
* file and include the License file at usr/src/OPENSOLARIS.LICENSE.
* If applicable, add the following below this CDDL HEADER, with the
* fields enclosed by brackets "[]" replaced with your own identifying
* information: Portions Copyright [yyyy] [name of copyright owner]
*
* CDDL HEADER END
*/
/*
* Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
* Copyright (c) 2011, 2018 by Delphix. All rights reserved.
* Copyright (c) 2017, Intel Corporation.
*/
#ifndef _SYS_METASLAB_H
#define _SYS_METASLAB_H
#include <sys/spa.h>
#include <sys/space_map.h>
#include <sys/txg.h>
#include <sys/zio.h>
#include <sys/avl.h>
#ifdef __cplusplus
extern "C" {
#endif
typedef struct metaslab_ops {
uint64_t (*msop_alloc)(metaslab_t *, uint64_t);
} metaslab_ops_t;
extern metaslab_ops_t *zfs_metaslab_ops;
int metaslab_init(metaslab_group_t *, uint64_t, uint64_t, uint64_t,
metaslab_t **);
void metaslab_fini(metaslab_t *);
void metaslab_set_unflushed_txg(metaslab_t *, uint64_t, dmu_tx_t *);
void metaslab_set_estimated_condensed_size(metaslab_t *, uint64_t, dmu_tx_t *);
uint64_t metaslab_unflushed_txg(metaslab_t *);
uint64_t metaslab_estimated_condensed_size(metaslab_t *);
int metaslab_sort_by_flushed(const void *, const void *);
uint64_t metaslab_unflushed_changes_memused(metaslab_t *);
int metaslab_load(metaslab_t *);
void metaslab_potentially_unload(metaslab_t *, uint64_t);
void metaslab_unload(metaslab_t *);
boolean_t metaslab_flush(metaslab_t *, dmu_tx_t *);
uint64_t metaslab_allocated_space(metaslab_t *);
void metaslab_sync(metaslab_t *, uint64_t);
void metaslab_sync_done(metaslab_t *, uint64_t);
void metaslab_sync_reassess(metaslab_group_t *);
uint64_t metaslab_block_maxsize(metaslab_t *);
/*
* metaslab alloc flags
*/
#define METASLAB_HINTBP_FAVOR 0x0
#define METASLAB_HINTBP_AVOID 0x1
#define METASLAB_GANG_HEADER 0x2
#define METASLAB_GANG_CHILD 0x4
#define METASLAB_ASYNC_ALLOC 0x8
#define METASLAB_DONT_THROTTLE 0x10
#define METASLAB_MUST_RESERVE 0x20
#define METASLAB_FASTWRITE 0x40
int metaslab_alloc(spa_t *, metaslab_class_t *, uint64_t,
blkptr_t *, int, uint64_t, blkptr_t *, int, zio_alloc_list_t *, zio_t *,
int);
int metaslab_alloc_dva(spa_t *, metaslab_class_t *, uint64_t,
dva_t *, int, dva_t *, uint64_t, int, zio_alloc_list_t *, int);
void metaslab_free(spa_t *, const blkptr_t *, uint64_t, boolean_t);
void metaslab_free_concrete(vdev_t *, uint64_t, uint64_t, boolean_t);
void metaslab_free_dva(spa_t *, const dva_t *, boolean_t);
void metaslab_free_impl_cb(uint64_t, vdev_t *, uint64_t, uint64_t, void *);
void metaslab_unalloc_dva(spa_t *, const dva_t *, uint64_t);
int metaslab_claim(spa_t *, const blkptr_t *, uint64_t);
int metaslab_claim_impl(vdev_t *, uint64_t, uint64_t, uint64_t);
void metaslab_check_free(spa_t *, const blkptr_t *);
void metaslab_fastwrite_mark(spa_t *, const blkptr_t *);
void metaslab_fastwrite_unmark(spa_t *, const blkptr_t *);
void metaslab_alloc_trace_init(void);
void metaslab_alloc_trace_fini(void);
void metaslab_trace_init(zio_alloc_list_t *);
void metaslab_trace_fini(zio_alloc_list_t *);
metaslab_class_t *metaslab_class_create(spa_t *, metaslab_ops_t *);
void metaslab_class_destroy(metaslab_class_t *);
int metaslab_class_validate(metaslab_class_t *);
void metaslab_class_histogram_verify(metaslab_class_t *);
uint64_t metaslab_class_fragmentation(metaslab_class_t *);
uint64_t metaslab_class_expandable_space(metaslab_class_t *);
boolean_t metaslab_class_throttle_reserve(metaslab_class_t *, int, int,
zio_t *, int);
void metaslab_class_throttle_unreserve(metaslab_class_t *, int, int, zio_t *);
uint64_t metaslab_class_get_alloc(metaslab_class_t *);
uint64_t metaslab_class_get_space(metaslab_class_t *);
uint64_t metaslab_class_get_dspace(metaslab_class_t *);
uint64_t metaslab_class_get_deferred(metaslab_class_t *);
void metaslab_space_update(vdev_t *, metaslab_class_t *,
int64_t, int64_t, int64_t);
metaslab_group_t *metaslab_group_create(metaslab_class_t *, vdev_t *, int);
void metaslab_group_destroy(metaslab_group_t *);
void metaslab_group_activate(metaslab_group_t *);
void metaslab_group_passivate(metaslab_group_t *);
boolean_t metaslab_group_initialized(metaslab_group_t *);
uint64_t metaslab_group_get_space(metaslab_group_t *);
void metaslab_group_histogram_verify(metaslab_group_t *);
uint64_t metaslab_group_fragmentation(metaslab_group_t *);
void metaslab_group_histogram_remove(metaslab_group_t *, metaslab_t *);
void metaslab_group_alloc_decrement(spa_t *, uint64_t, void *, int, int,
boolean_t);
void metaslab_group_alloc_verify(spa_t *, const blkptr_t *, void *, int);
void metaslab_recalculate_weight_and_sort(metaslab_t *);
void metaslab_disable(metaslab_t *);
void metaslab_enable(metaslab_t *, boolean_t);
extern int metaslab_debug_load;
#ifdef __cplusplus
}
#endif
#endif /* _SYS_METASLAB_H */
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