Re: lgwr/dbw0 did not issue pwrite64 sys call
- From: Sidney Chen <huanshengchen@xxxxxxxxx>
- To: "D'Hooge Freek" <Freek.DHooge@xxxxxxxxx>
- Date: Sun, 27 Nov 2011 13:28:28 +0800
It turn out it's because the ASMLIB is deployed on the server. AIO is
enable by disk_asynch_io set to true as default.
quote from note
*How To Check if Asynchronous I/O is Working On Linux [ID 237299.1]*
*ASM Inherently Performs Asynchronous I/O Regardless of
filesystemio_options Parameter [ID 751463.1]*
=== quote from 237299.1 ===
Caveat for ASMLibIf Oracle ASMLib (see
http://oss.oracle.com/projects/oracleasm ) is deployed, the *kiocb* structs
are not used. ASMLib does not use the POSIX aio_*() functions. You will
never see any *kioctx* or *kiocb* structures from ASMLib. It is far lower
level than that.
In fact, ASMLib does AIO or SyncIO depending on how the I/O is passed to
it, It makes no decisions at all. This is entirely up to kfk and the layers
above it, kfk is entirely controlled by the disk_asynch_io parameter. So,
we can check whether ASMLib is doing AIO by PL/SQL command "show param
disk_asynch_io". (You can disable AIO by setting disk_asynch_io=false)
With ASMLib, AIO is done via ioctl() calls (2.4 kernel), or *read()* calls
(2.6 kernel) on the ASM device. Whether ASMLib uses aio depends on whether
oracle is configured to do aio, In oracle 10g, if ASMLib is in use, the i/o
is asynchronous, because oracle 10g enables aio by default.
The strace when using ASMlib will show read calls that look like this:
...
*read(16, "MSA\0\2\0\10\0P\0\0\0\222\377\377\377@\313\373\5\0\0\0"..., 80) = 80*
...
The first 3 characters, byte-swapped, are ASM, indicating an ASMLib I/O
command structure.
--
Regards
Sidney Chen
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