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, instead
    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.

Thanks so much for taking a stab at this!

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