Re: [PATCH 1/5] btrfs: 1G falloc extents

[Wang Yugui <wangyugui@e16-tech.com>]

Hi,

> When doing a large fallocate, btrfs will break it up into 256MiB
> extents. Our data block groups are 1GiB, so a more natural maximum size
> is 1GiB, so that we tend to allocate and fully use block groups rather
> than fragmenting the file around.
> 
> This is especially useful if large fallocates tend to be for "round"
> amounts, which strikes me as a reasonable assumption.
> 
> While moving to size classes reduces the value of this change, it is
> also good to compare potential allocator algorithms against just 1G
> extents.


I wrote a 32G file, and the compare the result of 'xfs_io -c fiemap'.

dd conv=fsync bs=1024K count=32K if=/dev/zero of=/mnt/test/dd.txt

When write to a btrfs filesystem
+ xfs_io -c fiemap /mnt/test/dd.txt
/mnt/test/dd.txt:
        0: [0..262143]: 6883584..7145727
        1: [262144..524287]: 6367232..6629375
        2: [524288..8126463]: 7145728..14747903
        3: [8126464..8388607]: 15272064..15534207
        4: [8388608..8650751]: 14755840..15017983
        5: [8650752..16252927]: 15534208..23136383
        6: [16252928..67108863]: 23144448..74000383

When write to a xfs filesystem
+ xfs_io -c fiemap /mnt/test/dd.txt
/mnt/test/dd.txt:
        0: [0..16465919]: 256..16466175
        1: [16465920..31821623]: 16466176..31821879
        2: [31821624..41942903]: 31821880..41943159
        3: [41942904..58720111]: 47183872..63961079
        4: [58720112..67108863]: 63961080..72349831

the max of xfs is about 8G, but the max of btrfs is 
about 25G('6: [16252928..67108863]'?

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2022/10/07



> Signed-off-by: Boris Burkov <boris@bur.io>
> ---
>  fs/btrfs/inode.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 45ebef8d3ea8..fd66586ae2fc 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -9884,7 +9884,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
>  	if (trans)
>  		own_trans = false;
>  	while (num_bytes > 0) {
> -		cur_bytes = min_t(u64, num_bytes, SZ_256M);
> +		cur_bytes = min_t(u64, num_bytes, SZ_1G);
>  		cur_bytes = max(cur_bytes, min_size);
>  		/*
>  		 * If we are severely fragmented we could end up with really
> -- 
> 2.37.2