On 01/27/2012 12:21 AM, James Bottomley wrote:
On Thu, 2012-01-26 at 17:27 +0100, Bernd Schubert wrote:
On 01/26/2012 03:53 PM, Martin K. Petersen wrote:
"Bernd" == Bernd Schubert<bernd.schubert@xxxxxxxxxxxxxxxxxx> writes:
Bernd> We from the Fraunhofer FhGFS team would like to also see the T10
Bernd> DIF/DIX API exposed to user space, so that we could make use of
Bernd> it for our FhGFS file system. And I think this feature is not
Bernd> only useful for file systems, but in general, scientific
Bernd> applications, databases, etc also would benefit from insurance of
Bernd> data integrity.
I'm attending a SNIA meeting today to discuss a (cross-OS) data
integrity aware API. We'll see what comes out of that.
With the Linux hat on I'm still mainly interested in pursuing the
sys_dio interface Joel and I proposed last year. We have good experience
with that I/O model and it suits applications that want to interact with
the protection information well. libaio is also on my list.
But obviously any help and input is appreciated...
I guess you are referring to the interface described here
http://www.spinics.net/lists/linux-mm/msg14512.html
Hmm, direct IO would mean we could not use the page cache. As we are
using it, that would not really suit us. libaio then might be another
option then.
Are you really sure you want protection information and the page cache?
The reason for using DIO is that no-one could really think of a valid
page cache based use case. What most applications using protection
information want is to say: This is my data and this is the integrity
verification, send it down and assure me you wrote it correctly. If you
go via the page cache, we have all sorts of problems, like our
granularity is a page (not a block) so you'd have to guarantee to write
a page at a time (a mechanism for combining subpage units of protection
information sounds like a nightmare). The write becomes mark page dirty
and wait for the system to flush it, and we can update the page in the
meantime. How do we update the page and its protection information
atomically. What happens if the page gets updated but no protection
information is supplied and so on ... The can of worms just gets more
squirmy. Doing DIO only avoids all of this.
Well, entirely direct-IO will not work anyway as FhGFS is a parallel
network file system, so data are sent from clients to servers, so data
are not entirely direct anymore.
The problem with server side storage direct-IO is that it is too slow
for several work cases. I guess the write performance could be mostly
solved somehow, but then still the read-cache would be entirely missing.
From Lustre history I know that server side read-cache improved
performance of applications at several sites. So I really wouldn't like
to disable it for FhGFS...
I guess if we couldn't use the page cache, we probably wouldn't attempt
to use DIF/DIX interface, but will calculate our own checksums once we
are going to work on the data integrity feature on our side.
Cheers,
Bernd
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