jffs2 Erase Block Summary overhead

[Indrek Kruusa]

Ühel kenal päeval (neljapäev 15 veebruar 2007 5:30 pm) kirjutas Matthieu 
CASTET:
> Hi,
>
> after doing some test on a 64MB nand flash, we see that only 50 MB can
> be stored on the filesystem [1]. The extra 10 MB are taken by jffs2
> summary. If we disable jffs2 sumary in the kernel, the 10MB become
> available.

This is interesting. I have a Puppy Linux build within 52MB (ps 512B/ebs 16KB) 
and sumtool grows it to 57MB.
JFFS2/device with ps of 2048B/ebs 128KB has much much better compression level 
and sum adds ca. 3-4% for this case.
You can play with "dry" mkfs.jffs2 and different page;block sizes/file 
counts/file types to get the whole picture. Or read the code ;)
Somebody could put a nice summary table into FAQ with estimated results.

cheers,
Indrek



>
> Is there a way to reduce this overhead ?
> 20 % seems really big.
>
> May be Centralized Summary [3] give better result ?
> What is its status ?
>
> Matthieu
>
> [1]
> # time mount /dev/mtdblock3 -t jffs2 /tmp/mnt/ ; time cat
> /tmp/mnt/root/file.1 > /dev/null ; du -sh /tmp/mnt; df
> real    0m 3.05s
> user    0m 0.00s
> sys     0m 3.05s
> real    0m 0.34s
> user    0m 0.01s
> sys     0m 0.33s
> 50.6M   /tmp/mnt
> Filesystem                Size      Used Available Use% Mounted on
> /dev/mtdblock3           61.5M     60.7M    832.0k  99% /tmp/mnt
>
> [2]
> # time mount /dev/mtdblock3 -t jffs2 /tmp/mnt/ ; time cat
> /tmp/mnt/root/file.1 > /dev/null ; du -sh /tmp/mnt; df
> real    0m 11.30s
> user    0m 0.01s
> sys     0m 11.28s
> real    0m 0.35s
> user    0m 0.00s
> sys     0m 0.33s
> 50.6M   /tmp/mnt
> /dev/mtdblock3           61.5M     51.2M     10.3M  83% /tmp/mnt
>
> [3]
> http://www.inf.u-szeged.hu/jffs2/mount.php
>
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