Re: [PATCH v4 3/3] xfs: set aside allocation btree blocks from block reservation

[Chandan Babu R <chandanrlinux@gmail.com>]

On 23 Apr 2021 at 18:40, Brian Foster wrote:
> The blocks used for allocation btrees (bnobt and countbt) are
> technically considered free space. This is because as free space is
> used, allocbt blocks are removed and naturally become available for
> traditional allocation. However, this means that a significant
> portion of free space may consist of in-use btree blocks if free
> space is severely fragmented.
>
> On large filesystems with large perag reservations, this can lead to
> a rare but nasty condition where a significant amount of physical
> free space is available, but the majority of actual usable blocks
> consist of in-use allocbt blocks. We have a record of a (~12TB, 32
> AG) filesystem with multiple AGs in a state with ~2.5GB or so free
> blocks tracked across ~300 total allocbt blocks, but effectively at
> 100% full because the the free space is entirely consumed by
> refcountbt perag reservation.
>
> Such a large perag reservation is by design on large filesystems.
> The problem is that because the free space is so fragmented, this AG
> contributes the 300 or so allocbt blocks to the global counters as
> free space. If this pattern repeats across enough AGs, the
> filesystem lands in a state where global block reservation can
> outrun physical block availability. For example, a streaming
> buffered write on the affected filesystem continues to allow delayed
> allocation beyond the point where writeback starts to fail due to
> physical block allocation failures. The expected behavior is for the
> delalloc block reservation to fail gracefully with -ENOSPC before
> physical block allocation failure is a possibility.
>
> To address this problem, set aside in-use allocbt blocks at
> reservation time and thus ensure they cannot be reserved until truly
> available for physical allocation. This allows alloc btree metadata
> to continue to reside in free space, but dynamically adjusts
> reservation availability based on internal state. Note that the
> logic requires that the allocbt counter is fully populated at
> reservation time before it is fully effective. We currently rely on
> the mount time AGF scan in the perag reservation initialization code
> for this dependency on filesystems where it's most important (i.e.
> with active perag reservations).
>

The changes look good to me.

Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>

-- 
chandan