Re: [PATCH] libxfs: Get Physical Sector Size instead of Logical Sector size

From: Eric Sandeen <sandeen@sandeen.net>
To: Dave Chinner <david@fromorbit.com>
Cc: Carlos Maiolino <cmaiolino@redhat.com>, xfs@oss.sgi.com
Subject: Re: [PATCH] libxfs: Get Physical Sector Size instead of Logical Sector size
Date: Sun, 27 Nov 2011 17:05:23 -0600	[thread overview]
Message-ID: <4ED2C233.8010104@sandeen.net> (raw)
In-Reply-To: <20111127010643.GU2386@dastard>

On 11/26/11 7:06 PM, Dave Chinner wrote:
> On Thu, Nov 24, 2011 at 05:50:42PM -0200, Carlos Maiolino wrote:
>> On Thu, Nov 24, 2011 at 05:20:51PM -0200, Carlos Maiolino wrote:
>>> xfsprogs (mainly mkfs) is using the logical sector size of a volume to initialize
>>> the filesystem, which, even in devices using Advanced Format, it can get a 512
>>> bytes sector size if it is set as the logical sector size.
>>> This patch changes the ioctl to get the physical sector size, independent of the
>>> logical size.
>>>
>>
>> Just as information, this patch proposal does not change the behaviour of mkfs in case the 
>> user is using libblkid, which in case, mkfs will take advantage of libblkid to retrieve disk
>> topology and information.
>> I'm not sure if libblkid is the best way to retrieve the device sector's size here, since 
>> this does not provide a way to retrive the physical sector size, only the logical size, but 
>> I can be very wrong.
> 
> If libblkid exports the PBS (physical block size) as exposed in
> /sys/block/<dev>/queue/physical_block_size, then we should be able
> to get it.
> 
> However, the issue in my mind is not whether it is supported, but
> what is the effect of making this change? The filesystem relies on
> the fact that the minimum guaranteed unit of atomic IO is a sector,
> not the PBS of the underlying disk. What guarantees do we have when
> do a PBS sized IO is doesn't get torn into sector sized IOs by the
> drive and hence only partitially completed on failure?  Indeed, if
> the filesystem is sector unaligned, it is -guaranteed- to have PBS
> sized IOs torn apart by the hardware....
> 
> i.e. do we have any guarantee at all that a PBS sized IO will either
> wholly complete or wholly fail when PBS != sector size? And if not,
> why is this a change we should make given it appears to me to
> violate a fundamental assumption of the filesystem design?

I had the expectation that physical block size WAS the fundamental/atomic
IO size for the disk, and anything smaller required read/modify/write.
So I made this suggestion (and I think hch concurred) so that we weren't
doing log IOs which required RMW & translation.

i.e. for a 4k physical / 512 logical disk - wouldn't we want to choose
4k sectors?

Ok, if we have mismanaged the alignment and aligned to logical, not
physical, then I guess there would be an issue... but at that point
we've already messed up (though not catastrophically I guess)...

-Eric

> Cheers,
> 
> Dave.

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