On 02/22/2018 06:37 PM, Qu Wenruo wrote:
On 2018年02月23日 00:31, Ellis H. Wilson III wrote:
On 02/21/2018 11:56 PM, Qu Wenruo wrote:
On 2018年02月22日 12:52, Qu Wenruo wrote:
btrfs_read_block_groups() is used to build up the block group cache for
all block groups, so it will iterate all block group items in extent

For large filesystem (TB level), it will search for BLOCK_GROUP_ITEM
thousands times, which is the most time consuming part of mounting

So this patch will try to speed it up by:

1) Avoid unnecessary readahead
     We were using READA_FORWARD to search for block group item.
     However block group items are in fact scattered across quite a
lot of
     leaves. Doing readahead will just waste our IO (especially important
     for HDD).

     In real world case, for a filesystem with 3T used space, it would
     have about 50K extent tree leaves, but only have 3K block group
     items. Meaning we need to iterate 16 leaves to meet one block group
     on average.

     So readahead won't help but waste slow HDD seeks.

2) Use chunk mapping to locate block group items
     Since one block group item always has one corresponding chunk item,
     we could use chunk mapping to get the block group item size.

     With block group item size, we can do a pinpoint tree search,
     of searching with some uncertain value and do forward search.

     In some case, like next BLOCK_GROUP_ITEM is in the next leaf of
     current path, we could save such unnecessary tree block read.

Cc: Ellis H. Wilson III <ell...@panasas.com>

Hi Ellis,

Would you please try this patch to see if it helps to speedup the mount
of your large filesystem?

I will try either tomorrow or over the weekend.  I'm waiting on hardware
to be able to build and load a custom kernel on.

If you're using Archlinux, I could build the package for you.

(For other distributions, unfortunately I'm not that familiar with)


No sweat.  I'm not running arch anywhere, so was glad to handle this myself.

Short story: It doesn't appear to have any notable impact on mount time.

Long story:
#Built a modern kernel:
git clone https://github.com/kdave/btrfs-devel
cd'd into btrfs-devel
copied my current kernel config in /boot to .config
make olddefconfig
make -j16
make modules_install
make install
grub2-mkconfig -o /boot/grub/grub.cfg

#Reran tests with vanilla 4.16.0-rc1+ kernel
As root, of the form: time mount /dev/sdb /mnt/btrfs
5 iteration average: 16.869s

#Applied your patch, rebuild, switched kernel module
wget -O - 'https://patchwork.kernel.org/patch/10234619/mbox' | git am -
make -j16
make modules_install
rmmod btrfs
modprobe btrfs

#Reran tests with patched 4.16.0-rc1+ kernel
As root, of the form: time mount /dev/sdb /mnt/btrfs
5 iteration average: 16.642s

So, there's a slight improvement against vanilla 4.16.0-rc1+, but it's still slightly slower than my original runs in 4.5.5, which got me 16.553s. In any event, most of this is statistically unsignificant since the standard deviation is about two tenths of a second.

So, my conclusion here is this problem needs to be handled at an architectural level to be truly solved (read: have mounts that few seconds at worst), which either requires: a) On-disk format changes like you (Qu) suggested some time back for a tree of block groups or b) Lazy block group walking post-mount and algorithms that can cope with making sub-optimal choices. One would likely want to stonewall out certain operations until the lazy post-mount walk completed like balance, defrag, etc, that have more reason to require complete knowledge of the usage of each block group.

I may take a stab at b), but I'm first going to do the tests I promised relating to how mount times scale with increased capacity consumption for varying filesizes.


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