linux-ext4 - Re: [PATCH] ext4: check flags's EXT4_GET_BLOCKS_DELALLOC_RESERVE before call ext4_find_delalloc

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Message-ID: <CAGBYx2ZRo_XSnU0XAvmD1+ivARyOTdM1L76LNsve7CrtHrqVug@mail.gmail.com>
Date:	Wed, 7 Dec 2011 19:02:15 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Robin Dong <hao.bigrat@...il.com>
Cc:	linux-ext4@...r.kernel.org, Robin Dong <sanbai@...bao.com>
Subject: Re: [PATCH] ext4: check flags's EXT4_GET_BLOCKS_DELALLOC_RESERVE
 before call ext4_find_delalloc_cluster()

Hi Robin,

If a file system is mounted with delalloc and it changes to nodelalloc
mode thereafter, does the patch work?

Yongqiang.

On Wed, Dec 7, 2011 at 2:04 PM, Robin Dong <hao.bigrat@...il.com> wrote:
> From: Robin Dong <sanbai@...bao.com>
>
> We found performance regression when using bigalloc with "nodelalloc"  (1MB cluster size):
>
> 1. mke2fs -C 1048576 -O ^has_journal,bigalloc /dev/sda
> 2. mount -o nodelalloc /dev/sda /test/
> 3. time dd if=/dev/zero of=/test/io bs=1048576 count=1024
>
> The "dd" will cost about 2 seconds to finish, but if we mke2fs without "bigalloc",
> "dd" will only cost lesss than 1 second.
>
> The reason is:  when using ext4 with "nodelalloc", it will call ext4_find_delalloc_cluster() nearly
> everytime it call ext4_ext_map_blocks(), and ext4_find_delalloc_cluster() will also scan all pages
> in cluster because no buffer is "delayed".
> A cluster has 256 pages (1MB cluster), so it will scan 256 * 256k pags when creating a 1G file. That
> severely hurts the performance.
>
> Therefore, we don't call ext4_find_delalloc_cluster() when use "nodelalloc".
>
> Signed-off-by: Robin Dong <sanbai@...bao.com>
> ---
>  fs/ext4/extents.c |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 61fa9e1..e15d32b 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -3724,7 +3724,8 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
>        if (!(flags & EXT4_GET_BLOCKS_PUNCH_OUT_EXT) &&
>                ext4_ext_in_cache(inode, map->m_lblk, &newex)) {
>                if (!newex.ee_start_lo && !newex.ee_start_hi) {
> -                       if ((sbi->s_cluster_ratio > 1) &&
> +                       if ((flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE) &&
> +                           (sbi->s_cluster_ratio > 1) &&
>                            ext4_find_delalloc_cluster(inode, map->m_lblk, 0))
>                                map->m_flags |= EXT4_MAP_FROM_CLUSTER;
>
> @@ -3900,7 +3901,8 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
>                }
>        }
>
> -       if ((sbi->s_cluster_ratio > 1) &&
> +       if ((flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE) &&
> +           (sbi->s_cluster_ratio > 1) &&
>            ext4_find_delalloc_cluster(inode, map->m_lblk, 0))
>                map->m_flags |= EXT4_MAP_FROM_CLUSTER;
>
> --
> 1.7.4.1
>
> --
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-- 
Best Wishes
Yongqiang Yang
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