Re: [PATCH RFC 0/5] btrfs: scrub: introduce SCRUB_LOGICAL flag

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

On 2023/6/17 13:07, Qu Wenruo wrote:
> [BACKGROUND]
> Scrub originally works in a per-device basis, that means if we want to
> scrub the full fs, we would queue a scrub for each writeable device.
>
> This is normally fine, but some behavior is not ideal like the
> following:
> 		X	X+16K	X+32K
>   Mirror 1	|XXXXXXX|       |
>   Mirror 2	|       |XXXXXXX|
>
> When scrubbing mirror 1, we found [X, X+16K) has corruption, then we
> will try to read from mirror 2 and repair using the correct data.
>
> This applies to range [X+16K, X+32K) of mirror 2, causing the good copy
> to be read twice, which may or may not be a big deal.
>
> But the problem can easily go crazy for RAID56, as its repair requires
> the full stripe to be read out, so is its P/Q verification.
>
>
> There are also some other minor problems, like due to the difference in
> the disk read speed, we may be scrubbing different block groups on
> different devices.
> This can lead to reduced available space and higher chance of false
> -ENOSPC.
>
> [ENHANCEMENT]
> Instead of the existing per-device scrub, this patchset introduce a new
> flag, SCRUB_LOGICAL, to do logical address space based scrub.
>
> Unlike per-device scrub, this new flag would make scrub a per-fs
> operation.
>
> This allows us to scrub the different mirrors in one go, and avoid
> unnecessary read to repair the corruption.
>
> This also makes user space handling much simpler, just one ioctl to
> scrub the full fs, without the current multi-thread scrub code.
>
> [TODO]
> The long todo list is the main reason for RFC:
>
> - Missing RAID56 handling
>    Unlike pure mirror based profiles, RAID56 repair now needs to be
>    handled inside scrub (or some new raid interfaces to handle fully
>    cached stripes).
>
>    This can be some extra investment, and add an exception for the
>    elegant and simpler mirror based read-repair path.
>
> - Better progs integration
>    In theory we can silently try SCRUB_LOGICAL first, if the kernel
>    doesn't support it, then fallback to the old multi-device scrub.
>
>    But for current testing, a dedicated option is still assigned for
>    scrub subcommand.
>
>    And currently it only supports full report, no summary nor scrub file
>    support.
>
> - More testing
>    Currently only very basic tests done, no stress tests yet.

Forgot one more feature missing:

- IO speed limit through sysfs interface
   The old interface is per-device one, which can not be integrated
   easily into the per-fs scrub.

Thanks,
Qu

>
> Qu Wenruo (5):
>    btrfs: scrub: make scrub_ctx::stripes dynamically allocated
>    btrfs: scrub: introduce the skeleton for logical-scrub
>    btrfs: scrub: extract the common preparation before scrubbing a block
>      group
>    btrfs: scrub: implement the basic extent iteration code
>    btrfs: scrub: implement the repair part of scrub logical
>
>   fs/btrfs/disk-io.c         |   1 +
>   fs/btrfs/fs.h              |  12 +
>   fs/btrfs/ioctl.c           |   6 +-
>   fs/btrfs/scrub.c           | 757 ++++++++++++++++++++++++++++++-------
>   fs/btrfs/scrub.h           |   2 +
>   include/uapi/linux/btrfs.h |  11 +-
>   6 files changed, 647 insertions(+), 142 deletions(-)
>