02-17-2013
Hello Praveen,
I'm not sure if you are mixing FS layer and phisical (disk driver) layer. I was speaking about FS layer (and block device driver layer) in my first post.
mcnamara answer was right.
Quote:
Originally Posted by
Praveen_218
A block is still (on most FS) refers to 4K of data only.
On most system the page-cache is still of this fix sizes. Hence on all block I/O is of 4K or less. The file systems on such system utilizes the maximum size which is 4k.
However, if you see the disk architecture, they have been under trial by various vendors with various sector sizes; with 512bytes sector disk supported by most of the File system and storage product vendors (they however support various other size disks too -but 512b sector disk is in most common use probably because windows/DOS/UNIX FS supported them).
In order to support various disk architecture and FS supporting them use fragmentation which of course let you divide the 4k of page size into various fragments of 1, 2, ... 8 fragments per page.
8-fragments per page is the lowest value which translate into the size of a sector. You can of course not use a sector half of it. For I/O of 1 to full 512 byte of data a full sector gets used in one disk write.
Can you post here the steps you used to test this figures :
1) On UFS, how you saw the fragment size of 1 KB?
2) How did you looked at the 512kb I/O ?
1) fstype
2) dtrace script from DTrace Toolkit (bitesize.d)
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LEARN ABOUT OSX
bitesize.d
bitesize.d(1m) USER COMMANDS bitesize.d(1m)
NAME
bitesize.d - analyse disk I/O size by process. Uses DTrace.
SYNOPSIS
bitesize.d
DESCRIPTION
This produces a report for the size of disk events caused by processes. These are the disk events sent by the block I/O driver.
If applications must use the disks, we generally prefer they do so sequentially with large I/O sizes, or larger "bites".
Since this uses DTrace, only users with root privileges can run this command.
EXAMPLES
Sample until Ctrl-C is hit then print report,
# bitesize.d
FIELDS
PID process ID
CMD command and argument list
value size in bytes
count number of I/O operations
NOTES
The application may be requesting smaller sized operations, which are being rounded up to the nearest sector size or UFS block size.
To analyse what the application is requesting, DTraceToolkit programs such as Proc/fddist may help.
DOCUMENTATION
See the DTraceToolkit for further documentation under the Docs directory. The DTraceToolkit docs may include full worked examples with ver-
bose descriptions explaining the output.
EXIT
bitesize.d will sample until Ctrl-C is hit.
AUTHOR
Brendan Gregg [Sydney, Australia]
SEE ALSO
iosnoop(1M), seeksize(1M), dtrace(1M)
version 1.00 Jun 15, 2005 bitesize.d(1m)