Sorry, I missed your in-line reply:
1) The one right above, btrfs_write_out_cache, is the write-out of the
free space cache v1. Do you see this for multiple seconds going on, and
does it match the time when it's writing X MB/s to disk?
It seems to only last until the next watch update.
[<ffffffffaa0a8406>] io_schedule+0x16/0x40
[<ffffffffaa3b3cde>] get_request+0x23e/0x720
[<ffffffffaa3b6861>] blk_queue_bio+0xc1/0x3a0
[<ffffffffaa3b4a88>] generic_make_request+0xf8/0x2a0
[<ffffffffaa3b4ca5>] submit_bio+0x75/0x150
[<ffffffffc087fac5>] btrfs_map_bio+0xe5/0x2f0 [btrfs]
[<ffffffffc084834c>] btree_submit_bio_hook+0x8c/0xe0 [btrfs]
[<ffffffffc086f1e3>] submit_one_bio+0x63/0xa0 [btrfs]
[<ffffffffc086f39b>] flush_epd_write_bio+0x3b/0x50 [btrfs]
[<ffffffffc086f3be>] flush_write_bio+0xe/0x10 [btrfs]
[<ffffffffc08777a9>] btree_write_cache_pages+0x379/0x450 [btrfs]
[<ffffffffc08478ed>] btree_writepages+0x5d/0x70 [btrfs]
[<ffffffffaa1a326c>] do_writepages+0x1c/0x70
[<ffffffffaa196f2a>] __filemap_fdatawrite_range+0xaa/0xe0
[<ffffffffaa197023>] filemap_fdatawrite_range+0x13/0x20
[<ffffffffc084fba9>] btrfs_write_marked_extents+0xe9/0x110 [btrfs]
[<ffffffffc084fc4d>] btrfs_write_and_wait_transaction.isra.22+0x3d/0x80
[btrfs]
[<ffffffffc0851645>] btrfs_commit_transaction+0x665/0x900 [btrfs]
[<ffffffffc084baca>] transaction_kthread+0x18a/0x1c0 [btrfs]
[<ffffffffaa09b839>] kthread+0x109/0x140
[<ffffffffaa8459f5>] ret_from_fork+0x25/0x30
The last three lines will stick around for a while. Is switching to
space cache v2 something that everyone should be doing? Something that
would be a good test at least?
2) How big is this filesystem? What does your `btrfs fi df /mountpoint` say?
# btrfs fi df /export/
Data, single: total=30.45TiB, used=30.25TiB
System, DUP: total=32.00MiB, used=3.62MiB
Metadata, DUP: total=66.50GiB, used=65.08GiB
GlobalReserve, single: total=512.00MiB, used=53.69MiB
3) What kind of workload are you running? E.g. how can you describe it
within a range from "big files which just sit there" to "small writes
and deletes all over the place all the time"?
It's a pretty light workload most of the time. It's a file system that
exports two NFS shares to a small lab group. I believe it is more small
reads all over a large file (MRI imaging) rather than small writes.
4) What kernel version is this? `uname -a` output?
# uname -a
Linux machine_name 4.12.8-custom #1 SMP Tue Aug 22 10:15:01 EDT 2017
x86_64 x86_64 x86_64 GNU/Linux
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