[PATCH v1] erofs-utils: mkfs: Implement 'dsunit' alignment on blobdev

[Friendy.Su at sony.com]

Hi, Gao,

>So I tend to just constrain the case to your limited case
>first, could you explain more if chunk deduplication is
>needed for your scenarios? and what's your real `chunksize`?

chunk deduplication is needed. 

As I wrote in commit msg, we expect scenario below:

1. mount with -o dax=always, 
2. application calls mmap(addr, file).
3. application read from addr, page fault is triggered. 
We hope in kernel, erofs_dax_vm_ops.huge_fault() can be handled, do not fallback to erofs_dax_vm_ops.fault().

This required file body on blob devices aligned on huge page(2M), and deduplicate unit also is 2M. We can specify --dsunit=512, --chunksize=2*1024*1024 to fulfill this.

I don't think need a new command option. 
Currently, '--dsunit' can be set for formatting blobdev. The following cmdline completes successfully. User certainly thinks mkfs.erofs has executed --dsunit alignment. 
But actually, it does not.  This patch just simply makes actual runtime fit for cmdline looks like.

mkfs.erofs --blobdev /dev/sdb1 --dsunit 512 ......

If actually `--dsunit` does not work on blobdev, should prompt warning msg to user.

Best Regards
Friendy Su

________________________________________
From: Gao Xiang <hsiangkao at linux.alibaba.com>
Sent: Thursday, August 21, 2025 10:00
To: Su, Friendy; linux-erofs at lists.ozlabs.org
Cc: Mo, Yuezhang; Palmer, Daniel (SGC)
Subject: Re: [PATCH v1] erofs-utils: mkfs: Implement 'dsunit' alignment on blobdev

On 2025/8/20 17: 38, Friendy. Su@ sony. com wrote: > Hi, Gao, > >> What's your `--chunksize` ? consider the following: > > chunksize = 4096 > dsunit = 512 = 2M > >> and two inodes: > >> inode A (8k) 2M, 2M+4k




On 2025/8/20 17:38, Friendy.Su at sony.com wrote:
> Hi, Gao,
>
>> What's your `--chunksize` ? consider the following:
>
>    chunksize = 4096
>    dsunit = 512 = 2M
>
>> and two inodes:
>
>> inode A (8k)    2M, 2M+4k
>
>> inode B (12k)   4M, 2M, 4M+4k, 4M+8k?
>
>> Is it possible? what's the expected behavior of
>> this case.
>
> Yes. This is the expected behavior. See runtime below:

I understand that is the expected behavior according to this
patch, but I'm just unsure if it's an expected behavior for
the future wider setups (because some users may use `--dsunit`
for other usage).

So I tend to just constrain the case to your limited case
first, could you explain more if chunk deduplication is
needed for your scenarios? and what's your real `chunksize`?

Maybe adding another command option for this is better.

Thanks,
Gao Xiang