inode checkpoints
Artem B. Bityuckiy
dedekind at mail.ru
Sat Oct 9 09:22:32 EDT 2004
Artem B. Bityuckiy wrote:
> Hello Guys.
>
> Unfortunately, I hit on very painful trouble while trying to
> design/implement checkpoints (ICP) ...
>
> Obviously, ICP must contain all information, which is required to build
> the inode cache. These are: offset, size and *version*. The problem is
> in version.
>
> At first I thought that in order to create ICP for some regfile inode,
> we just need to be sure that the inode cache for this regfile exists.
> But! I've overseen the fact that *there is no versions in the inode
> cache* :-(
>
> The inode cache (struct jffs2_inode_info) contains fragtree which has
> struct jffs2_full_dnode objects. But these objects have no version field...
>
> So, I see two ways out:
>
> 1. Add the version field to the struct jffs2_full_dnode objects.
> This also means that the struct jffs2_tmp_dnode_info structure won't be
> needed anymore.
>
> The advantage of such approach is that the JFFS2 will be more simple
> since one data structure will be removed. I like the KISS principle.
Ok, guys, sorry, I've hurried saying that the struct
jffs2_tmp_dnode_info won't be needed... It is needed to construct lists
of struct jffs2_full_dnode objects...
Anyway, my problem is still actual.
>
> The drawback is that the inode cache will eat more memory. But this
> isn't in-core object, just cache, so I don't think this is a big
> disadvantage.
>
> 2. Don't use the inode cache at all. This is bad because in order to
> build the inode cache we'll need to read *all* the node headers, even if
> there is existing inode cache.
>
> I believe this approach is bad and too heavyweight. Moreover, just
> imagine the situation when the GC has fount an ICP ant wants to
> determine if it valid or obsolete. Obsolete means that (1) there is
> newer ICP with higher version (simple case) or (2) all (or most) the
> nodes which are described by the ICP aren't valid anymore (complicated
> case). The second case means that we must read the ICPs lowest_version
> and highest_version and count how many valid nodes with versions within
> this interval exist. But we have NO versions even if the inode cache is
> present. I don't think it is a good idea if the GC will head all the
> node's headers...
>
> What do you guys think? Any advices/ideas?
>
> Thanks.
>
--
Best Regards
Artem B. Bityckiy
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