Re: "O/S block size" for Windows NT

Ben Pomicter wrote:
>
> Here's a seemingly simple question often asked:
> What is the "O/S block size" for Windows NT?
>
> Everyone in this forum, and many at Oracle support for that matter,
> seem to have their own answer. As for Microsoft's documentation -- the
> MSDN Library and Technet -- it *never* makes use of the term "block
> size" in any discussion of file systems or datafile I/O, nor does it use
> the phrases "O/S block size" or "operating system block" in any context.
>
> There are generally two type of answers given:
> 1) some absolute value -- usually 2048 bytes
> 2) whatever the cluster size is -- which can be 512, 1024, 2048, etc.,
> up to 64k, and is set (manually or by default) when you format the
> volume
>
> I'm assuming the following things in this discussion: Oracle8, NT4,
> NTFS. Also, keep in mind the difference between sectors and clusters.
>
> I've done some experimenting myself using the init.ora parameters
> log_checkpoint_interval and max_dump_file_size, which are (or can be)
> expressed as a number of "operating system blocks." For example, set
> log_checkpoint_interval=1024 and log_checkpoints_to_alert=true, then
> later on check the alert log and see how many checkpoints there are per
> log switch. Say your log files are 20M, and you're getting 40
> checkpoints per log switch, then one "operating system block" is
> (20M/40)/1024=512 bytes. This is the result I keep getting, 512 bytes,
> and this is on volumes with a 4k cluster size. 512 bytes is the
> *sector* size of the disk.
>
> The answers given generally equate an "O/S block" with the atomic unit
> of disk I/O used by NT. However, there is no reference in Microsoft's
> documentaton (nor in any of the other, many, books and articles I've
> scoured) that NT does I/O in, say, 2048-byte units. Nor is there any
> reference that a cluster is anything more than a unit of disk space
> allocation, as opposed to a unit of I/O.
>
> In addition, there is documentation that Oracle (and SQL Server for that
> matter) actually does datafile I/O on the sector level:
> " On UNIX, Oracle opens files using the O_SYNC flag to bypass the
> filesystem buffer cache. In the current Win32 API, the equivalent flags
> are FILE_FLAG_NO_BUFFERING and FILE_FLAG_WRITE_THROUGH. The goal in both
> cases is the same; to ensure that data has been posted to disk before
> assuming that the write has been successful." [source: Oracle Metalink,
> Doc ID 48681.1, "Server Architecture on UNIX and NT"]
> "The file system cache is used, by default, whenever a disk is accessed.
> However, applications can request that its files not be cached by using
> the FILE_FLAG_NO_BUFFERING parameter in its call to open a file. This is
> called unbuffered I/O. Applications that use unbuffered I/O are
> typically database applications (such as SQL Server) [and Oracle] that
> manage their own cache buffers. Unbuffered I/O requests must be issued
> in multiples of the disk sector size"[source: NT Workstation 4.0
> Resource Kit, "Chapter 15 - Detecting Cache Bottlenecks"]
> "In SQL Server 7.0, pages [i.e. db blocks] are 8 KB, while Windows NT
> does I/O in 512-byte segments." [source: MSDN Library, "Microsoft SQL
> Server 7.0 Storage Engine"]
> There are lots of other references to this "unbuffered I/O" and the fact
> that it is in units of the disk sector size. Here is what Oracle
> support has said:
> "Oracle uses non-buffered reads and writes on NT. We therefore bypass
> the NT file system cache and are thus only concerned with I/O in
> multiples of the O/S blocksize (512 bytes)...Cluster sizes come into
> play simply as an allocation unit...As far as Oracle is concerned (and
> NT for that matter), its datafiles are already created...So if we want
> to read a 2k Oracle block we pass this to the O/S who simply does a 2k
> read (4 sectors) from the existing file and passes the results back to
> Oracle and our own buffer cache. The allocation unit (cluster) has no
> affect on this."
>
> Note that if 512 bytes is the unit of I/O, then the whole idea that
> db_block_size should be a multiple of the "O/S block size" is a moot
> point. (Remember, I'm only talking about NT here. For a JFS-based
> database on AIX, for example, Oracle does say that db_block_size should
> be a multiple of the JFS blocksize, 4k, while AIX does also have a
> physical block size of 512 bytes).
>
> So, here's what I'm asking: can any of you out there, who feel that the
> NTFS cluster size, or something else, is the low-level unit of
> datafile I/O for Oracle on NT, come up with any chapter-and-verse
> references which substantiate that assertion? I would love to see them
> for myself, as, I'm sure, would a lot of other people.
>
> --
> Ben Pomicter
> Database Administrator
> Digitas
>
> Sent via Deja.com http://www.deja.com/
> Before you buy.

I think Oracle uses direct I/O on NT, which should not be affected by the NT block sizes.

HTH

-- 
===========================================
Connor McDonald
http://www.oracledba.co.uk

We are born naked, wet and hungry...then things get worse