[Fwd: Re: a problem about jffs2]

[zhao, forrest]

> > Dear Linus:
> > 
> > 	I recently meet a problem about jffs2.
> > 
> > 	There have three sectors in my mobilephone(OMAP730):
> > 	/dev/mtdblock3           16384      3276     13108  20% /System
> > 	/dev/mtdblock4            2048       388      1660  19% /Sysdisk
> > 	/dev/mtdblock5            5120       388      4732   8% /Appdisk
> > 
> > 	I built new file(ph_bk) in /Sysdisk dirctory. 	I write a test
> > program, as follows:
> > 	1) open it.
> > 	fd = fopen(argv[1], "r+");
> > 	2) fseek with a random number, then write a number
> > 	for (;;) {
> > 		offset = rand();
> > 		fseek(fd, offset % (1024*1024), SEEK_SET);
> > 		fwrite(&offset,sizeof(int), 1, fd);
> > 	}
> > 
> > 	I want to limit the file with 1M size, but when the test program run
> > in background,
> > 	this file maintain 1M size, but available space on /dev/mtdblock4
> > continue to reduce, as below:
> > 	Filesystem           1k-blocks      Used Available Use% Mounted on
> > 	/dev/mtdblock4            2048       512      1536  25% /Sysdisk
> > /dev/mtdblock4            2048       688      1360  34% /Sysdisk
> > 	/dev/mtdblock4            2048       792      1256  39% /Sysdisk
> > 	/dev/mtdblock4            2048       856      1192  42% /Sysdisk
> > 	/dev/mtdblock4            2048      1212       836  59% /Sysdisk
> > 	/dev/mtdblock4            2048      1516       532  74% /Sysdisk
> > 	/dev/mtdblock4            2048      1920       128  94% /Sysdisk
> > 
> > 	Is it jffs2 filesystem's bug?
> > 
I once did some testing like yours, and had the same result.

I would say that this is an intrinsic nature of JFFS2 instead of a
JFFS2's bug.
Updating 4 bytes of a file for every write operation will cause
flash space wasting since every write operation generate an 
jffs2_raw_inode + 4 bytes on flash. sizeof(jffs2_raw_inode) is 68 bytes.

Thanks,
Forrest