Re: Rebuild mode for tail-pack layouts

Hi xtex,

On 2026/4/30 20:09, xtex wrote:
> Hi!
> 
> In erofs_rebuild_write_blob_index (rebuild.c), only EROFS_INODE_CHUNK_BASED
> and FLAT_PLAIN are implemented, so when generating a metadata index with
> rebuild mode, the sources cannot use tail-pack nor inline data layout.
> However, disabling tail-packing can lead to great disk-space waste in many
> cases, especially when the file-system consists of a lot of small files.
> 
> Thus I attempted to implement FLAT_INLINE for it, only to realize that the
> current chunk entry formats can only represent physical addresses that are
> block-aligned while tail-pack extent is not.
> 
> I wonder what do you think about adding a new chunk entry format? And how
> should it be named?
> 
> I would suggest the following structure:
> struct erofs_inode_chunk_index_tp {
> 	__le16 startblk_hi;	/* starting block number MSB */
> 	__le16 device_id;	/* back-end storage id (with bits masked)
> */
> 	__le32 startblk_lo;	/* starting block number of this chunk */
> 	/* new fields below */
> 	__le16 startblk_off;	/* starting block offset */
> 	__le16 reserved;
> } __packed;
> The 16b offset should be enough unless we are to support block size > 64K.
> The reserved field is added for alignment.

Sorry about the late response.

Thanks for the question.

FLAT_INLINE can be used for index rebuilding, which can work with
uniaddr (or mapped_blkaddr) since the blkaddr will be mapped
into the relative address based on the blob starting with
mapped_blkaddr:

https://erofs.docs.kernel.org/en/latest/ondisk/chunked_format.html#device-table

But I agree the expression in the page above is a bit
ambigious through.

Thanks,
Gao Xiang


> 
> Best wishes.
>