Inaccurate description of statx(2) stx_blksize field?

Inaccurate description of statx(2) stx_blksize field?

[Zeno Endemann]

Hi,

Currently, the documentation of stx_blksize in statx says:

 > The "preferred" block size for efficient filesystem I/O.
 > (Writing to a file in smaller chunks may cause an
 > inefficient read-modify-rewrite.)

I believe this is misleading; For ext4 this field is set
to the ext4 block size, but in that context "block size"
refers to the "smallest allocation unit" (which is chosen
at file system creation time, see the mkfs.ext4 -b option)
and not the most efficient I/O size that avoids the need
for read-modify-rewrite.

At least to my understanding, to avoid such a read-modify-
rewrite you rather want to do writes in multiples of the
page size (i.e. sysconf(PAGESIZE)), since that is the unit
the page cache operates on.

While in many cases ext4 will have a block size equal to
the page size, I think it is strictly better to use the
page size directly for that purpose. Or am I mistaken?


Thanks,
Zeno Endemann

Re: Inaccurate description of statx(2) stx_blksize field?

[Alejandro Colomar]

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[CC += Ted]

Hi,

On 2026-04-22T12:37:43+0200, Zeno Endemann wrote:
> Hi,
> 
> Currently, the documentation of stx_blksize in statx says:
> 
> > The "preferred" block size for efficient filesystem I/O.
> > (Writing to a file in smaller chunks may cause an
> > inefficient read-modify-rewrite.)
> 
> I believe this is misleading; For ext4 this field is set
> to the ext4 block size, but in that context "block size"
> refers to the "smallest allocation unit" (which is chosen
> at file system creation time, see the mkfs.ext4 -b option)
> and not the most efficient I/O size that avoids the need
> for read-modify-rewrite.
> 
> At least to my understanding, to avoid such a read-modify-
> rewrite you rather want to do writes in multiples of the
> page size (i.e. sysconf(PAGESIZE)), since that is the unit
> the page cache operates on.
> 
> While in many cases ext4 will have a block size equal to
> the page size, I think it is strictly better to use the
> page size directly for that purpose. Or am I mistaken?

I don't know.  You should CC someone expert about file systems when
asking fs questions.


Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es>

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