Re: Periodic frame losses when recording to btrfs volume with OBS

[Chris Mason <clm@fb.com>]

On 01/22/2018 01:33 PM, Sebastian Ochmann wrote:

[ skipping to the traces ;) ]

> 2866 ffmpeg-mux D
> [<ffffffffc06251b1>] btrfs_start_ordered_extent+0x101/0x130 [btrfs]
> [<ffffffffc061b0b0>] lock_and_cleanup_extent_if_need+0x340/0x380 [btrfs]
> [<ffffffffc061c3c1>] __btrfs_buffered_write+0x261/0x740 [btrfs]
> [<ffffffffc06202bf>] btrfs_file_write_iter+0x20f/0x650 [btrfs]
> [<ffffffff9b235309>] __vfs_write+0xf9/0x170
> [<ffffffff9b23551d>] vfs_write+0xad/0x1a0
> [<ffffffff9b235762>] SyS_write+0x52/0xc0
> [<ffffffff9b800083>] entry_SYSCALL_64_fastpath+0x1a/0x7d
> [<ffffffffffffffff>] 0xffffffffffffffff

This is where we wait for writes that are already in flight before we're 
allowed to redirty those pages in the file.  It'll happen when we either 
overwrite a page in the file that we've already written, or when we're 
trickling down writes slowly in non-4K aligned writes.

You can probably figure out pretty quickly which is the case by stracing 
ffmpeg-mux.  Since lower dirty ratios made it happen more often for you, 
my guess is the app is sending down unaligned writes.

-chris