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Special Forums Hardware DD command using block device as input Post 302666117 by nytty on Tuesday 3rd of July 2012 06:03:41 PM
Old 07-03-2012
At the application level page size is an important concept, it's the unit of interaction with the disk. Imagine a database system requesting a record A, it'd be completely inefficient to only read this record (couple of bytes), instead we collocate a bunch of records in a page and we bet on spatial/temporal locality ...
It's a tradeof, if you set the page size to too big, you risk over reading stuff you dont need. Too small, and you risk to get multiple requests for otherwise contiguous records. A bit what the os does with 4096b pages.
I am trying to study that, using a buffered read of different sizes and calculate the throughtput ... I know only basics about disks, but i am certain that the metric i am studying is not uniform, thus doing the math is just meaningless.
I should cope with all parameters(cache,vm,readahead..ect) and have something like:
bs=512b 0.01sec 50kb/s
bs=16kb 0.0004sec 164mb/s
bs=2M 0.01sec 200mb/s
Then i will decide that 16kb is the best time vs throughtput trade off and this will be my page size.
 

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VFS_READAHEAD(8)														  VFS_READAHEAD(8)

NAME
vfs_readahead - pre-load the kernel buffer cache SYNOPSIS
vfs objects = readahead DESCRIPTION
This VFS module is part of the samba(7) suite. This vfs_readahead VFS module detects read requests at multiples of a given offset (hex 0x80000 by default) and then tells the kernel via either the readahead system call (on Linux) or the posix_fadvise system call to pre-fetch this data into the buffer cache. This module is useful for Windows Vista clients reading data using the Windows Explorer program, which asynchronously does multiple file read requests at offset boundaries of 0x80000 bytes. The offset multiple used is given by the readahead:offset option, which defaults to 0x80000. The size of the disk read operations performed by vfs_readahead is determined by the readahead:length option. By default this is set to the same value as the readahead:offset option and if not set explicitly will use the current value of readahead:offset. This module is stackable. OPTIONS
readahead:offset = BYTES The offset multiple that causes readahead to be requested of the kernel buffer cache. readahead:length = BYTES The number of bytes requested to be read into the kernel buffer cache on each readahead call. The following suffixes may be applied to BYTES: o K - BYTES is a number of kilobytes o M - BYTES is a number of megabytes o G - BYTES is a number of gigabytes EXAMPLES
[hypothetical] vfs objects = readahead VERSION
This man page is correct for version 3.0.25 of the Samba suite. AUTHOR
The original Samba software and related utilities were created by Andrew Tridgell. Samba is now developed by the Samba Team as an Open Source project similar to the way the Linux kernel is developed. VFS_READAHEAD(8)
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