[support-gang] jffs2 vs ubifs vs ext4 space-efficiency question

James Cameron quozl at laptop.org
Tue Dec 22 16:28:31 EST 2015

Yes, jffs2 compresses data.  That's why it is so slow.  That's why SD
card is faster than it should be otherwise.

No, there's no commonly used compression for ext4.

It would be a performance tradeoff; compression takes time and power.

On Tue, Dec 22, 2015 at 04:24:48PM -0500, Adam Holt wrote:
> Any ideas why Release 13.2.6 placed on ext4 instead of jffs2 appears to almost
> double in size?
> Starting with 13.2.6 on XO-1 on jffs2 per usual, "du -s /* | sort -nr" yields
> these contents for a stock/vanilla XO-1:
>    1070992 /usr
>    239060  /home
>    117623  /var
>    18780   /etc
>    15382   /boot
>    8735    /versions
>    1032    /run
>    8        /tmp
>    ...
> Naively summing up the above numbers gives a figure of almost 1.5GB, and yet we
> all know the XO-1 is limited to 1.0GB :-)  Is jffs2 somehow compressing the
> above files by about 2X, which are clearly contained within 769MB as shown in
> "df -hT" below?
>    Filesystem  Type  Size  Used  Avail   Use% Mounted on
>    mtd0           jffs2  1.0G  769M  256M  76%  /
>    ...
> Basic tests putting 13.2.6 on an SD card with ext4 show a rough doubling in
> size with the same files (du's ~1.5GB of files indeed sum to about 1.5GB when
> listed using df), which I presume indicates ext4 dispenses with compression,
> unlike the more space-efficient to jffs2 (and presumably ubifs) etc designed
> for flash/SD devices ?
> Apologies for my naivete: Would any ext4 partitioning options (or ubifs
> instead) offer any functionality similar to this apparently huge compression
> win offered by jffs2?  (Given I'm told jffs2 is not appropriate for the larger
> SD cards I'm experimenting with, even before we get to wear-leveling
> questions!)

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James Cameron

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