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    Subject[PATCH] ext4: Change the handling of RAID stripe width
    Date
    Previously, the stripe width was blindly used for determining the size
    of allocations. Now, the stripe width is used as a hint for the initial
    mb_group_prealloc; if it is greater than 1, then we make sure that
    mb_group_prealloc is some multiple of it, and otherwise it is ignored.
    mb_group_prealloc is always usable to adjust the preallocation strategy,
    not just when the stripe-width is 0 as before.

    Signed-off-by: Dan Ehrenberg <dehrenberg@google.com>
    ---
    fs/ext4/mballoc.c | 40 +++++++++++++++++++++++++++++-----------
    1 files changed, 29 insertions(+), 11 deletions(-)

    diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
    index 6ed859d..710c27f 100644
    --- a/fs/ext4/mballoc.c
    +++ b/fs/ext4/mballoc.c
    @@ -127,13 +127,14 @@
    * based on file size. This can be found in ext4_mb_normalize_request. If
    * we are doing a group prealloc we try to normalize the request to
    * sbi->s_mb_group_prealloc. Default value of s_mb_group_prealloc is
    - * 512 blocks. This can be tuned via
    - * /sys/fs/ext4/<partition/mb_group_prealloc. The value is represented in
    - * terms of number of blocks. If we have mounted the file system with -O
    + * 512 blocks. If we have mounted the file system with -O
    * stripe=<value> option the group prealloc request is normalized to the
    - * stripe value (sbi->s_stripe)
    + * the smallest multiple of the stripe value (sbi->s_stripe) which is
    + * greater than the default mb_group_prealloc. This can be tuned via
    + * /sys/fs/ext4/<partition>/mb_group_prealloc. The value is represented in
    + * terms of number of blocks.
    *
    - * The regular allocator(using the buddy cache) supports few tunables.
    + * The regular allocator (using the buddy cache) supports a few tunables.
    *
    * /sys/fs/ext4/<partition>/mb_min_to_scan
    * /sys/fs/ext4/<partition>/mb_max_to_scan
    @@ -2471,7 +2472,26 @@ int ext4_mb_init(struct super_block *sb, int needs_recovery)
    sbi->s_mb_stats = MB_DEFAULT_STATS;
    sbi->s_mb_stream_request = MB_DEFAULT_STREAM_THRESHOLD;
    sbi->s_mb_order2_reqs = MB_DEFAULT_ORDER2_REQS;
    + /*
    + * If the stripe width is 1, this makes no sense and
    + * we set it to 0 to turn off stripe handling code.
    + */
    + if (sbi->s_stripe == 1)
    + sbi->s_stripe = 0;
    +
    sbi->s_mb_group_prealloc = MB_DEFAULT_GROUP_PREALLOC;
    + /*
    + * If there is a s_stripe > 1, then we set the s_mb_group_prealloc
    + * to the lowest multiple of s_stripe which is bigger than
    + * the s_mb_group_prealloc as determined above. We want
    + * the preallocation size to be an exact multiple of the
    + * RAID stripe size so that preallocations don't fragment
    + * the stripes.
    + */
    + if (sbi->s_stripe > 1) {
    + sbi->s_mb_group_prealloc = roundup(
    + sbi->s_mb_group_prealloc, sbi->s_stripe);
    + }

    sbi->s_locality_groups = alloc_percpu(struct ext4_locality_group);
    if (sbi->s_locality_groups == NULL) {
    @@ -2830,8 +2850,9 @@ out_err:

    /*
    * here we normalize request for locality group
    - * Group request are normalized to s_strip size if we set the same via mount
    - * option. If not we set it to s_mb_group_prealloc which can be configured via
    + * Group request are normalized to s_mb_group_prealloc, which goes to
    + * s_strip if we set the same via mount option.
    + * s_mb_group_prealloc can be configured via
    * /sys/fs/ext4/<partition>/mb_group_prealloc
    *
    * XXX: should we try to preallocate more than the group has now?
    @@ -2842,10 +2863,7 @@ static void ext4_mb_normalize_group_request(struct ext4_allocation_context *ac)
    struct ext4_locality_group *lg = ac->ac_lg;

    BUG_ON(lg == NULL);
    - if (EXT4_SB(sb)->s_stripe)
    - ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_stripe;
    - else
    - ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_mb_group_prealloc;
    + ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_mb_group_prealloc;
    mb_debug(1, "#%u: goal %u blocks for locality group\n",
    current->pid, ac->ac_g_ex.fe_len);
    }
    --
    1.7.3.1


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