Thank you for the comment. I just figured it out and was checking ext4. This problem never happens with ext4 !!! It's from ext3 indirect block addressing. Thank you Bart. On Sun, Dec 2, 2012 at 7:26 PM, Bart Van Assche <bvanassche@xxxxxxx> wrote: > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
