Re: [PATCH 0/7] erofs: introduce long xattr name prefixes feature

[Chao Yu <chao@kernel.org>]

On 2023/4/7 22:17, Jingbo Xu wrote:
> Background
> =========
> overlayfs uses xattrs to keep its own metadata.  If such xattrs are
> heavily used, such as Composefs model [1], large amount of xattrs
> with diverse xattr values exist but only a few common xattr names
> are valid (trusted.overlay.redirect, trusted.overlay.digest, and
> maybe more in the future) . For example:
> 
> # file 1
> trusted.overlay.redirect="xxx"
> 
> # file 2
> trusted.overlay.redirect="yyy"
> 
> That makes the existing predefined prefixes ineffective in both image
> size and runtime performance.
> 
> Solution Proposed
> ====================
> Let's introduce long xattr name prefixes now to fix this.  They work
> similarly as the predefined name prefixes, except that long xattr name
> prefixes are user specified.
> 
> When a long xattr name prefix is used, the shared long xattr prefixes
> are stored in the packed or meta inode, while the remained trailing part
> of the xattr name apart from the long xattr name prefix will be stored
> in erofs_xattr_entry.e_name.  e_name is empty if the xattr name matches
> exactly as the long xattr name prefix.
> 
> Erofs image sizes will be smaller in the above described scenario where
> all files have diverse xattr values with the same set of xattr names[2],
> such as having following xattrs like:
> 
> trusted.overlay.metacopy=""
> trusted.overlay.redirect="xxx"
> 
> Here are the image sizes for comparison (32-byte inodes are used):
> 
> ```
> 7.4M  large.erofs.T0.xattr
> 6.4M  large.erofs.T0.xattr.share
> ```
> 
> - share: "--xattr-prefix=trusted.overlay.redirect" option of mkfs.erofs.
> w/ this option, the long xattr name prefixes feature is enabled.
> 
> It can be seen ~14% disk space is saved with this feature in this
> typical workload, therefore metadata I/Os could also be more effective
> then.
> 
> Test
> ====
> It passes erofs/019 of erofs-utils.
> 
> 
> [1] https://lore.kernel.org/all/CAOQ4uxgGc33_QVBXMbQTnmbpHio4amv=W7ax2vQ1UMet0k_KoA@mail.gmail.com/
> [2] https://my.owndrive.com/index.php/s/irHJXRpZHtT3a5i
> 
> 
> 
> Gao Xiang (1):
>    erofs: keep meta inode into erofs_buf
> 
> Jingbo Xu (6):
>    erofs: initialize packed inode after root inode is assigned
>    erofs: move packed inode out of the compression part
>    erofs: introduce on-disk format for long xattr name prefixes
>    erofs: add helpers to load long xattr name prefixes
>    erofs: handle long xattr name prefixes properly
>    erofs: enable long extended attribute name prefixes

Acked-by: Chao Yu <chao@kernel.org>

Thanks,