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

[Johannes Thumshirn <jthumshirn@suse.de>]

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On 08/10/2019 11:26, Qu Wenruo wrote:
> 
> 
> 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.
> 

Ah thanks for the explanation. I'll give your testscript a try.


-- 
Johannes Thumshirn                            SUSE Labs Filesystems
jthumshirn@suse.de                                +49 911 74053 689
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