From: "Wilcox, Matthew R" <matthew.r.wilcox@intel.com>
To: Jeff Moyer <jmoyer@redhat.com>, Linda Knippers <linda.knippers@hp.com>
Cc: Boaz Harrosh <boaz@plexistor.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"Verma, Vishal L" <vishal.l.verma@intel.com>
Subject: RE: regression introduced by "block: Add support for DAX reads/writes to block devices"
Date: Thu, 13 Aug 2015 19:32:10 +0000 [thread overview]
Message-ID: <100D68C7BA14664A8938383216E40DE040915418@FMSMSX114.amr.corp.intel.com> (raw)
In-Reply-To: <x49mvxv3vc6.fsf@segfault.boston.devel.redhat.com>
I liked the patch you were pushing to request the *page* containing the requested bytes instead of the *block* containing the requested bytes.
For the misaligned partition problem, I was thinking we should change the direct_access API to return a phys_addr_t instead of a pfn. That way we can return something that isn't actually page aligned, and DAX can take care of making sure it doesn't overshoot the end.
-----Original Message-----
From: Jeff Moyer [mailto:jmoyer@redhat.com]
Sent: Thursday, August 13, 2015 11:19 AM
To: Linda Knippers
Cc: Boaz Harrosh; Wilcox, Matthew R; linux-kernel@vger.kernel.org; linux-fsdevel@vger.kernel.org; Verma, Vishal L
Subject: Re: regression introduced by "block: Add support for DAX reads/writes to block devices"
Linda Knippers <linda.knippers@hp.com> writes:
>>> It causes the physical block size to be PAGE_SIZE but the
>>> logical block size is still 512. However, the minimum_io_size
>>> is now 4096 (same as physical block size, I assume). The
>>> optimal_io_size is still 0. What does that mean?
>>
>> physical block size - device's internal block size
>> logical block size - addressable unit
>
> Right, but it's still reported as 512 and that doesn't work.
Understood. :)
>> optimal io size - device's preferred unit for streaming
>
> So 0 is ok.
Correct.
>> We can change the block device to export logical/physical block sizes of
>> PAGE_SIZE. However, when persistent memory support comes to platforms
>> that support page sizes > 32k, xfs will again run into problems (Dave
>> Chinner mentioned that xfs can't deal with logical block sizes >32k.)
>> Arguably, you can use pmem and dax on such platforms using RAM today for
>> testing. Do we care about breaking that?
>
> I would think so. AARCH64 uses 64k pages today.
So does powerpc, but I guess nobody cares about that anymore. ;-) If
the logical block size is smaller than the page size, we're going to
have to deal with sub-page I/O. For now, we can do as Boaz suggested,
and just turn off dax for those configurations. We could also just
revert the patch that introduced this problem. I really don't know who
is going to care about O_DIRECT I/O performance to a persistent memory
block device.
Willy? What was the real motivation there?
> I think Documentation/filesystems/dax.txt could use a little update
> too. It has a section "Implementation Tips for Block Driver Writers"
> that makes it sound easy but now I wonder if it even works with the
> example ram drivers. Should we be able to read any 512 byte
> "sector"?
If the logical block size is 512 bytes, then you have to be able to do
(direct) I/O to any 512 byte sector. Simple as that.
Cheers,
Jeff
next prev parent reply other threads:[~2015-08-13 19:32 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-05 20:19 regression introduced by "block: Add support for DAX reads/writes to block devices" Jeff Moyer
2015-08-05 22:01 ` Dave Chinner
2015-08-06 1:42 ` Linda Knippers
2015-08-06 3:24 ` Dave Chinner
2015-08-06 7:52 ` Boaz Harrosh
2015-08-06 20:34 ` Dave Chinner
2015-08-09 8:52 ` Boaz Harrosh
2015-08-10 16:32 ` Linda Knippers
2015-08-10 21:27 ` Dave Chinner
2015-08-10 23:04 ` Linda Knippers
2015-08-06 14:21 ` Wilcox, Matthew R
2015-08-06 15:33 ` Jeff Moyer
2015-08-06 15:51 ` Wilcox, Matthew R
2015-08-06 21:30 ` Jeff Moyer
2015-08-07 18:11 ` Wilcox, Matthew R
2015-08-07 20:41 ` Jeff Moyer
2015-08-10 7:42 ` Boaz Harrosh
2015-08-12 21:11 ` Jeff Moyer
2015-08-13 5:32 ` Boaz Harrosh
2015-08-13 14:00 ` Jeff Moyer
2015-08-13 16:42 ` Linda Knippers
2015-08-13 17:14 ` Jeff Moyer
2015-08-13 17:52 ` Linda Knippers
2015-08-13 18:19 ` Jeff Moyer
2015-08-13 19:32 ` Wilcox, Matthew R [this message]
2015-08-14 16:28 ` Dan Williams
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