Re: [PATCH] mkfs: Remove messages printed when blocksize < physical sectorsize

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On 9/5/17 5:10 PM, Dave Chinner wrote:
> On Tue, Sep 05, 2017 at 09:17:42AM -0500, Eric Sandeen wrote:
>>
>>
>> On 9/5/17 1:44 AM, Dave Chinner wrote:
>>> On Mon, Sep 04, 2017 at 11:31:39PM -0700, Christoph Hellwig wrote:
>>>> On Tue, Sep 05, 2017 at 11:14:42AM +0530, Chandan Rajendra wrote:
>>>>> Linux kernel commit 6c6b6f28b3335fd85ec833ee0005d9c9dca6c003 (loop: set
>>>>> physical block size to PAGE_SIZE) now sets PAGE_SIZE as the default
>>>>> physical sector size of loop devices. On ppc64, this causes loop devices
>>>>> to have 64k as the physical sector size.
>>>>
>>>> Eek.  We'll need to revert the loop change ASAP!
>>>
>>> And, FWIW, making the warning go away if probably a bad idea,
>>> because XFS only supports devices with sector sizes up
>>> to 32k:
>>
>> Well, TBH removing this warning was my suggestion, because it's
>> automatically fixing values that weren't specified by the user in
>> the first place.  First preference is physical sector size, then
>> fallback to logical but it doesn't need to be noisy about it.
>>
>>> #define XFS_MIN_SECTORSIZE_LOG  9       /* i.e. 512 bytes */
>>> #define XFS_MAX_SECTORSIZE_LOG  15      /* i.e. 32768 bytes */
>>>
>>> And so it should be warning about devices trying to tell it to use
>>> something larger....
>>
>> As long as the logical sector size is small enough, it seems like a
>> silent adjustment is probably ok,  no?
> 
> Think 512e drives. Doing 512 byte sector IO is possible, but slow.
> Someone might actually want to avoid that by having the filesystem
> use 4k sector sizes. However, if for some reason, mkfs selects 512
> byte sectors (the logical size) rather than 4k sector size, then
> shouldn't we be telling the user we're doing something that has a
> "for-the-life-of-the-filesystem" performance impact?

Well, sure, but it'll only select 512 if it /has/ to, i.e. if the
block size is < 4k.

So for the simple case of a 512e drive, our default block size is
4k, physical size is 4k, and everything is happy and fine.

If the user /specifies/ a 1k block size on such a device, how much
of a nanny do we really want to be about telling them this is suboptimal?

There are a lot of suboptimal things you can specify on the
mkfs commandline, but we don't generally choose to warn about them...

-Eric

> Cheers,
> 
> Dave.
> 



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