Re: [PATCH v2 0/3] btrfs: Introduce new incompat feature BG_TREE to hugely reduce mount time

[Qu Wenruo]

On 2019/10/8 下午5:14, Johannes Thumshirn wrote:
>> [[Benchmark]]
>> Since I have upgraded my rig to all NVME storage, there is no HDD
>> test result.
>>
>> Physical device:     NVMe SSD
>> VM device:           VirtIO block device, backup by sparse file
>> Nodesize:            4K  (to bump up tree height)
>> Extent data size:    4M
>> Fs size used:                1T
>>
>> All file extents on disk is in 4M size, preallocated to reduce space usage
>> (as the VM uses loopback block device backed by sparse file)
> 
> Do you have a some additional details about the test setup? I tried to
> do the same (testing) for a bug Felix (added to Cc) reported to my at
> the ALPSS Conference and I couldn't reproduce the issue.
> 
> My testing was a 100TB sparse file passed into a VM and running this
> script to touch all blockgroups:

Here is my test scripts:
---
#!/bin/bash

dev="/dev/vdb"
mnt="/mnt/btrfs"

nr_subv=16
nr_extents=16384
extent_size=$((4 * 1024 * 1024)) # 4M

_fail()
{
        echo "!!! FAILED: $@ !!!"
        exit 1
}

fill_one_subv()
{
        path=$1
        if [ -z $path ]; then
                _fail "wrong parameter for fill_one_subv"
        fi
        btrfs subv create $path || _fail "create subv"

        for i in $(seq 0 $((nr_extents - 1))); do
                fallocate -o $((i * $extent_size)) -l $extent_size
$path/file || _fail "fallocate"
        done
}

declare -a pids
umount $mnt &> /dev/null
umount $dev &> /dev/null

#~/btrfs-progs/mkfs.btrfs -f -n 4k $dev -O bg-tree
mkfs.btrfs -f -n 4k $dev
mount $dev $mnt -o nospace_cache

for i in $(seq 1 $nr_subv); do
        fill_one_subv $mnt/subv_${i} &
        pids[$i]=$!
done

for i in $(seq 1 $nr_subv); do
        wait ${pids[$i]}
done
sync
umount $dev

---

> 
> #!/bin/sh
> 
> FILE=/mnt/test
> 
> add_dirty_bg() {
>         off="$1"
>         len="$2"
>         touch $FILE
>         xfs_io -c "falloc $off $len" $FILE
>         rm $FILE
> }
> 
> mkfs.btrfs /dev/vda
> mount /dev/vda /mnt
> 
> for ((i = 1; i < 100000; i++)); do
>         add_dirty_bg $i"G" "1G"
> done

This wont really build a good enough extent tree layout.

1G fallocate will only cause 8 128M file extents, thus 8 EXTENT_ITEMs.

Thus a leaf (16K by default) can still contain a lot of BLOCK_GROUPS all
together.

To build a case to really show the problem, you'll need a lot of
EXTENT_ITEM/METADATA_ITEMS to fill the gaps between BLOCK_GROUPS.

My test scripts did that, but may still not represent the real world, as
real world can cause even smaller extents due to snapshots.

Thanks,
Qu

> 
> umount /mnt
> 
> 
> 

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