Re: Moving db to linux

On 02/28/2004 08:09:19 PM, Nuno Souto wrote: >> ----- Original Message -----

>>    if (stat3=posix_memalign(&buff,ALIGN,BUFFSIZE)) {
>>       fprintf(stderr,"ALIGN ERR:%s\n",strerror(stat3));
>>       exit(0);
>>    }

> :D I LIKE that! Would you believe this is the FIRST time EVER
> I've seen this little piece of code included in ANY program that
> purports to test I/O? Yet it is of capital importance: it makes a
> HUGE difference in getting any performance numbers. Well done!

Nuno, I agree, it's absolutely crucial because without the proper alignment, direct I/O will not work. If you look into open(2), for O_DIRECT flag, it does say that buffers need to be aligned on the buffer size. I am running the latest 2.4 kernel, and if I declare buffer as buff[BUFFSIZE], I get EINVAL for the next I/O operation. Exactly the same thing happens to Oracle 10 when I set the DIRECTIO flag. I did strace of CKPT process, and that was precisely what I saw. I chose CKPT because oracle was reporting problems with accessing control files when I did "alter database mount". I must humbly admit contacting Dave Kleinkamp from IBM team which is developing JFS, and it was he who has put the posix_memalign, not me. Below is his message:

• Dave Kleikamp <shaggy_at_austin.ibm.com> wrote:
  > Subject: Re: JFS, Linux and direct I/O
  > From: Dave Kleikamp <shaggy_at_austin.ibm.com>
  > To: Mladen Gogala <mgogala_at_adelphia.net>
  > Date: Fri, 27 Feb 2004 08:38:57 -0600
  >
  > On Thu, 2004-02-26 at 23:13, Mladen Gogala wrote:
  > > Hi!
  > > Few months ago, I asked about JFS and direct I/O and I received the
  > > answer that JFS on Linux supports direct I/O. Oracle doesn't think so and
  > > I ventured to write a little test program on my RH-9, with kernel 2.4.25.
  > > Test program is attached and it inevitably fails with O_DIRECT flags and
  > > works without them. How exactly can I use direct I/O with JFS? The fact
  > > that IBM claims that direct I/O is supported was the primary reason for me
  >
  > > to chose JFS. Please, explain me how can I utilize direct I/O from JFS
  > > files? In the mean time, I'll start testing XFS.
  > > Thanks!
  >
  > The problem is with your test case. The buffer passed to read and write
  > must be aligned to the file system's block size. From the open(2) man
  > page:
  >
  > "Transfer sizes, and the alignment of user buffer and file offset must
  > all be multiples of the logical block size of the file system."
  >
  > Attached is a modified version of the test case which succeeds on the
  > 2.4.25 kernel.
  

The modified version is the one that you saw earlier.

-- 
Mladen Gogala
Oracle DBA
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