Re: [PATCH v2 1/2] xfstests: introduce 279 for SEEK_DATA/SEEK_HOLE sanity check

[Jeff Liu <jeff.liu@oracle.com>]

On 02/10/2012 01:20 PM, Dave Chinner wrote:

> On Fri, Feb 10, 2012 at 11:24:26AM +0800, Jeff Liu wrote:
>> On 02/10/2012 06:25 AM, Dave Chinner wrote:
>>
>>> On Thu, Feb 09, 2012 at 10:27:05PM +0800, Jeff Liu wrote:
>>>> Strange, I also tried to build XFS with 2k which shown as following:
>>>>
>>>> $ sudo mkfs.xfs -b size=2k -n size=2k -f /dev/sda7
>>>>
>>>> $ xfs_info /dev/sda7
>>>> meta-data=/dev/sda7              isize=256    agcount=4, agsize=1418736 blks
>>>>          =                       sectsz=512   attr=2
>>>> data     =                       bsize=2048   blocks=5674944, imaxpct=25
>>>                                    ^^^^^^^^^^
>>>
>>>>          =                       sunit=0      swidth=0 blks
>>>> naming   =version 2              bsize=2048   ascii-ci=0
>>>                                    ^^^^^^^^^^
>>>
>>>> log      =internal               bsize=2048   blocks=5120, version=2
>>>                                    ^^^^^^^^^^
>>> The block size for data, metadata, directories and the log is 2k,
>>> just like you asked.
>>
>> Sorry, I mislead you.
>>
>> Yes, the block size for data and metadata, etc are ok for me, but the
>> allocate unit at "struct stat.st_blksize" is 4k, It should match
>> data->bsize=2k IMHO.
> 
> That field has nothing to do with the filesystem block size.
> According to the stat(2) man page:
> 
> 'The st_blksize field gives the "preferred" blocksize for efficient
> file system I/O.'
> 
> Giving a value of less than PAGE_SIZE for this field leads to
> inefficient IO because it forces the page cache to do
> read-modify-write cycles for single filesystem block writes. Hence
> on a 4k page size machine, it needs to report 4k as a minimum to
> avoid this. On a 64k page size machine, you'll find that value is
> 64k.

Sigh, I was misled by EXT4's output for stat(2), since its st_blksize is
2k which is equal to the mkfs formating value even on a 4k page size
machine. :(

> 
> Indeed, XFS gives you some control over what is actually reported
> here. If your file lies on a real-time device, then XFS will export
> the extent allocation size (either the mkfs default of the per inode
> hint if it is set) in this field. For files on the data device, if
> you mount with the "largeio" mount option, XFS will export the
> stripe width if it is set, the biosize if that mount option is used
> or the PAGE_SIZE if neither are set. These are all different
> but valid definitions of "preferred blocksize for efficient IO".
> 
> If you want to know the real block size of the filesystem, use
> statfs(2).

Definitely, thanks a lot!!

-Jeff

> 
> Cheers,
> 
> Dave.


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