Re: one 16K random read I/O issues 2 scsi I/O (16K and 4K) requests

[Bart Van Assche <bvanassche@acm.org>]

On 12/02/12 10:23, Hiroyuki Yamada wrote:
> I figured out what is going on, but I don't know what it is for.
>
> Ext3 filesystem has some 4KB data in each 4096KB(8192 sectors) data.
> Visually, data is aligned like the following.
>
> |4KB|4096KB|4KB|4096KB|4KB|4096KB| ...
>
> And 4096KB area in only accessible by application programs.
> When accessing the first 4096KB area for the first time,
> then OS reads the 4KB just before the 4096KB area first
> and then read the requested data in the 4096KB area.
>
> When accessing a large file (compared to the DRAM size) randomly,
> every I/O has rare chance of hitting page cahce,
> so every I/O request comes together with 4KB I/O.
>
> The thing is what the 4KB data is for ?
> Is this location metadata for filesystem ?
> Is there any way I can remove this ?
> Or Is there any way I can clear the 4096KB area only ?

Does this behavior also occur with ext4 ? From the ext4 wiki 
(http://ext4.wiki.kernel.org/index.php/Ext4_Howto#Extents):

  Extents

Traditional, Unix-derived, file systems, like Ext3, use a indirect block 
mapping scheme to keep track of each block used for the blocks 
corresponding to the data of a file. This is inefficient for large 
files, especially during large file delete and truncate operations, 
because the mapping keeps an entry for every single block, and big files 
have many blocks -> huge mappings, slow to handle. Modern file systems 
use a different approach called "extents". An extent is basically a 
bunch of contiguous physical blocks. It basically says "The data is in 
the next n blocks". For example, a 100 MiB file can be allocated into a 
single extent of that size, instead of needing to create the indirect 
mapping for 25600 blocks (4 KiB per block). Huge files are split in 
several extents. Extents improve the performance and also help to reduce 
the fragmentation, since an extent encourages continuous layouts on the 
disk.

Bart.