Re: ASM parameters

From: Steve Howard <>
Date: Sun, 27 Jan 2008 15:00:21 -0800 (PST)
Message-ID: <>

On Jan 27, 4:17 pm, Mladen Gogala <> wrote:
> On Sat, 26 Jan 2008 19:14:14 -0800, Charles Hooper wrote:
> > "Take the Guesswork Out of Database Layout and I/O Tuning with Automatic
> > Storage Management"
> >
> 20guesswork%20out%20of%20db%20tuning%2001-06.pdf
> > "The Kernel Sequential File I/O (ksfq) provides support for sequential
> > disk/tape access and buffer management. Ksfq allocates 4 sequential
> > buffers by default. The size of the buffers is determined by
> > dbfile_direct_io_count parameter set to 1MB by default. Some of the
> > ksfq clients are Datafile, Redolog file, RMAN, Archive log file,
> > Datapump, Data Guard and the File Transfer Package."
> Charles, this is a great document. Here is my brief understanding how
> ASM works. ASM runs in user space, not in kernel space, which means it
> isn't a driver. It only provides database processes (s00x, dbwr, lgwr,
> ckpt) with locations where to read from or write to. The IO calls
> themselves are still performed by the corresponding database processes
> and are targeted to the underlying raw devices.
> In other words, ASM handles what is known as "file system metadata" -
> directories, files, extent maps and alike. If you take a look at IBM
> JFS, the open source implementation, you will spot the terms "transaction"
> and "file system metadata" in the include headers. JFS is, of course, a
> full grown file system with various options. ASM is not, but Oracle still
> needs to know where exactly on the disk block 20A3F in the file 133 is.
> ASM takes care of that. DBA puts a bunch of raw devices into an ASM disk
> group and ASM creates extent map and "directories" for him.
> On the operating system level, one can control caching of file blocks and
> prefetch. I was trying to do the same with ASM, but to no avail. One very
> nice thing about general purpose cluster file system like JFS is that one
> can open files using the O_DIRECT flag and without it. If the file is
> opened without the O_DIRECT flag, it will be buffered and the file system
> prefetch will be applied to it. That will be of great help for utilities
> like tar, cpio, cp, scp or gzip and performance of these utilities will be
> as expected. If, on the other hand, ASM or OCFS is all you have, better be
> prepared for a shock. Simple tar or cp operations will take hours,
> literally. Not even the oracle version of those utilities will help much.
> Buffering and prefetch are the only cure. That is why I moved
> log_archive_dest outside of ASM, wherever possible. Still, the performance
> of RMAN backup is abysmal. I am using RMAN with the MML library for
> NetBackup. With the log_archive_dest (ext3 cross-mounted using NFS3) I am
> getting 30 MB/sec transfer rate. With ASM - only 5MB sec. I am alleviating
> the problem by using "BACKUP AS COMPRESSED BACKUPSET", but that, too, is
> slow. I was hoping for an under the hood file system cache and prefetch
> implementation.
> --

Hi Mladen,

I appreciate this analysis, as it is difficult to get this information from Oracle in a straight forward, plain ole' English manner sometimes. I will say that I have seen really good throughput moving ASM files with dbms_file_transfer, using both the copy_file (local) and put_file (remote) procedures.


Steve Received on Sun Jan 27 2008 - 17:00:21 CST

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