JFFS2: real free capacity of the partition

Hello,

Probably this is a trivial question and won't take much of your time.

A simple test on a JFFS2 partition:

# df
Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/mtdblock1            1024       376       648  37% /etc
# dd if=/dev/urandom of=tmp bs=1024 count=2900
dd: tmp: No space left on device
# rm tmp
# df
Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/mtdblock1            1024       372       652  36% /etc

Why additional space appeared?

I've noted, that this effect:
- appeared only on the well-used partition, not on the fresh-created one.
- appeared only after filling up the device
- appeared only once. On subsequent tests, the free space was reported without magic increases.

Is this is an effect of the JFFS2 garbage collector? Is there any method to learn the real available capacity of the mounted partition?

Thanks in advance,
Sergei

Hello all,

Seems I found why is that happens.
In jffs2_statfs() the wasted_size is excluded
from calculation:
...
avail = c->dirty_size + c->free_size;
...

Could someone explain for what reason?
Have I understand it right - on the certain conditions, the wasted space can be converted to dirty space?

Regards,
Sergei

Wasted space is just the same as dirty space, really. The only difference is that we don't expect it to be reclaimed by garbage collection. The small spaces at the end of an eraseblock, for example, are still likely to be there when we write a new eraseblock full of data. There'll still be a small amount of wasted space at the end of the new block.

That's why we treat 'wasted' space differently from dirty space -- otherwise the GC might just keep going for ever and ever, trying to reclaim 'dirty' space but always not quite managing to reclaim as much as it needs.

dwmw2