Re: [PATCH v2 0/3] raid1 balancing methods

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2024/10/11 13:19, Anand Jain 写道:
> v2:
> 1. Move new features to CONFIG_BTRFS_EXPERIMENTAL instead of CONFIG_BTRFS_DEBUG.
> 2. Correct the typo from %est_wait to %best_wait.
> 3. Initialize %best_wait to U64_MAX and remove the check for 0.
> 4. Implement rotation with a minimum contiguous read threshold before
>     switching to the next stripe. Configure this, using:
>
>          echo rotation:[min_contiguous_read] > /sys/fs/btrfs/<uuid>/read_policy
>
>     The default value is the sector size, and the min_contiguous_read
>     value must be a multiple of the sector size.
>
> 5. Tested FIO random read/write and defrag compression workloads with
>     min_contiguous_read set to sector size, 192k, and 256k.
>
>     RAID1 balancing method rotation is better for multi-process workloads
>     such as fio and also single-process workload such as defragmentation.
>
>       $ fio --filename=/btrfs/foo --size=5Gi --direct=1 --rw=randrw --bs=4k \
>          --ioengine=libaio --iodepth=256 --runtime=120 --numjobs=4 \
>          --time_based --group_reporting --name=iops-test-job --eta-newline=1

Reviewed-by: Qu Wenruo <wqu@suse.com>

Although not 100% happy with the min_contiguous_read setting, since it's
an optional one and still experimental, I'm fine with series so far.


Just want to express my concern about going mount option.

I know sysfs is not a good way to setup a lot of features, but mount
option is way too committed to me, even under experimental features.

But I also understand without mount option it can be pretty hard to
setup the read policy for fstests runs.

So I'd prefer to have some on-disk solution (XATTR or temporary items)
to save the read policy.
It's less committed compared to mount option (aka, much easier to revert
the change with breaking any compatibility), and can help for future
features.

Thanks,
Qu
>
>
> |         |            |            | Read I/O count  |
> |         | Read       | Write      | devid1 | devid2 |
> |---------|------------|------------|--------|--------|
> | pid     | 20.3MiB/s  | 20.5MiB/s  | 313895 | 313895 |
> | rotation|            |            |        |        |
> |     4096| 20.4MiB/s  | 20.5MiB/s  | 313895 | 313895 |
> |   196608| 20.2MiB/s  | 20.2MiB/s  | 310152 | 310175 |
> |   262144| 20.3MiB/s  | 20.4MiB/s  | 312180 | 312191 |
> |  latency| 18.4MiB/s  | 18.4MiB/s  | 272980 | 291683 |
> | devid:1 | 14.8MiB/s  | 14.9MiB/s  | 456376 | 0      |
>
>     rotation RAID1 balancing technique performs more than 2x better for
>     single-process defrag.
>
>        $ time -p btrfs filesystem defrag -r -f -c /btrfs
>
>
> |         | Time  | Read I/O Count  |
> |         | Real  | devid1 | devid2 |
> |---------|-------|--------|--------|
> | pid     | 18.00s| 3800   | 0      |
> | rotation|       |        |        |
> |     4096|  8.95s| 1900   | 1901   |
> |   196608|  8.50s| 1881   | 1919   |
> |   262144|  8.80s| 1881   | 1919   |
> | latency | 17.18s| 3800   | 0      |
> | devid:2 | 17.48s| 0      | 3800   |
>
> Rotation keeps all devices active, and for now, the Rotation RAID1
> balancing method is preferable as default. More workload testing is
> needed while the code is EXPERIMENTAL.
> While Latency is better during the failing/unstable block layer transport.
> As of now these two techniques, are needed to be further independently
> tested with different worloads, and in the long term we should be merge
> these technique to a unified heuristic.
>
> Rotation keeps all devices active, and for now, the Rotation RAID1
> balancing method should be the default. More workload testing is needed
> while the code is EXPERIMENTAL.
>
> Latency is smarter with unstable block layer transport.
>
> Both techniques need independent testing across workloads, with the goal of
> eventually merging them into a unified approach? for the long term.
>
> Devid is a hands-on approach, provides manual or user-space script control.
>
> These RAID1 balancing methods are tunable via the sysfs knob.
> The mount -o option and btrfs properties are under consideration.
>
> Thx.
>
> --------- original v1 ------------
>
> The RAID1-balancing methods helps distribute read I/O across devices, and
> this patch introduces three balancing methods: rotation, latency, and
> devid. These methods are enabled under the `CONFIG_BTRFS_DEBUG` config
> option and are on top of the previously added
> `/sys/fs/btrfs/<UUID>/read_policy` interface to configure the desired
> RAID1 read balancing method.
>
> I've tested these patches using fio and filesystem defragmentation
> workloads on a two-device RAID1 setup (with both data and metadata
> mirrored across identical devices). I tracked device read counts by
> extracting stats from `/sys/devices/<..>/stat` for each device. Below is
> a summary of the results, with each result the average of three
> iterations.
>
> A typical generic random rw workload:
>
> $ fio --filename=/btrfs/foo --size=10Gi --direct=1 --rw=randrw --bs=4k \
>    --ioengine=libaio --iodepth=256 --runtime=120 --numjobs=4 --time_based \
>    --group_reporting --name=iops-test-job --eta-newline=1
>
> |         |            |            | Read I/O count  |
> |         | Read       | Write      | devid1 | devid2 |
> |---------|------------|------------|--------|--------|
> | pid     | 29.4MiB/s  | 29.5MiB/s  | 456548 | 447975 |
> | rotation| 29.3MiB/s  | 29.3MiB/s  | 450105 | 450055 |
> | latency | 21.9MiB/s  | 21.9MiB/s  | 672387 | 0      |
> | devid:1 | 22.0MiB/s  | 22.0MiB/s  | 674788 | 0      |
>
> Defragmentation with compression workload:
>
> $ xfs_io -f -d -c 'pwrite -S 0xab 0 1G' /btrfs/foo
> $ sync
> $ echo 3 > /proc/sys/vm/drop_caches
> $ btrfs filesystem defrag -f -c /btrfs/foo
>
> |         | Time  | Read I/O Count  |
> |         | Real  | devid1 | devid2 |
> |---------|-------|--------|--------|
> | pid     | 21.61s| 3810   | 0      |
> | rotation| 11.55s| 1905   | 1905   |
> | latency | 20.99s| 0      | 3810   |
> | devid:2 | 21.41s| 0      | 3810   |
>
> . The PID-based balancing method works well for the generic random rw fio
>    workload.
> . The rotation method is ideal when you want to keep both devices active,
>    and it boosts performance in sequential defragmentation scenarios.
> . The latency-based method work well when we have mixed device types or
>    when one device experiences intermittent I/O failures the latency
>    increases and it automatically picks the other device for further Read
>    IOs.
> . The devid method is a more hands-on approach, useful for diagnosing and
>    testing RAID1 mirror synchronizations.
>
> Anand Jain (3):
>    btrfs: introduce RAID1 round-robin read balancing
>    btrfs: use the path with the lowest latency for RAID1 reads
>    btrfs: add RAID1 preferred read device
>
>   fs/btrfs/disk-io.c |   4 ++
>   fs/btrfs/sysfs.c   | 116 +++++++++++++++++++++++++++++++++++++++------
>   fs/btrfs/volumes.c | 109 ++++++++++++++++++++++++++++++++++++++++++
>   fs/btrfs/volumes.h |  16 +++++++
>   4 files changed, 230 insertions(+), 15 deletions(-)
>