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Re: JFFS2 empty file overhead questions




jlavi@xxxxxxx.fi said:
> jffs2_inode_cache  25002  25012     20  148  148    1
> jffs2_node_frag        0      0     20    0    0    1
> jffs2_raw_node_ref  50063  50096     16  248  248    1
> jffs2_tmp_dnode        0      0     12    0    0    1
> jffs2_raw_inode        0      0     68    0    0    1> 
> jffs2_raw_dirent       0     92     40    0    1   > 1
> jffs2_full_dnode       1    202     16    1    1  >   1

OK, so you have ~1MiB of jffs2_raw_node_refs and ~0.5MiB of 
jffs2_inode_caches. Each file you've created has two nodes associated with 
it, and an entry in the inode hash table.

Without that bare minimum of information, we wouldn't be able to open the 
file on demand -- we'd have to scan the whole medium for nodes belonging to 
each file on read_inode(). There's very little we can do about it.

> size-64            25009  25016     64  424  424    1

Hmmm. Those will be the associated 25000-odd jffs2_full_dirents, each 
containing the name. That's because you have the directory inode in-core, 
and it has a complete list of the associated dirents and names. That's most 
of another 2MiB.

If we want to optimise for this case, perhaps we could stop keeping the 
name in memory. That would reduce the size of the object to 21 bytes, and 
we have a hash of the name anyway which we use for pre-comparison so we 
wouldn't need to read the actual name from the flash very often in 
lookup(), and readdir() can just live with it.

We could probably drop some other members from the jffs2_full_dirent while 
we're at it - given that we're going to have to read from the flash in 
lookup() and readdir() there's not a lot of point in keeping the version, 
ino# and type either -- so we may be able to get that down to 12 bytes per 
dirent, which would be only 400KiB.

We'd then need to invent a 'jffs2_tmp_dirent_info' with all the original 
information in it again, for use by the scan code much like the 
jffs2_tmp_dnode_info -- because we don't want to have to go back to the 
medium again during scan.

--
dwmw2



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