On Tue, 10 Apr 2001, Matt Dillon wrote:
> :> I'm not 100% convinced about the algorithm to avoid clusters filling
> :> up with directory-only entries (it looks like a worst-case would fill
> :> a cluster with 50% directories and 50% files leaving a bad layout when
> :> the directories are populated further), but then the non-dirpref
> :> scheme has some far worse worst-case scenarios ;-)
> :
> :Just to follow up on myself... it seems the dirpref stuff was
> :committed to FreeBSD this morning :-]
> :
> :--
> :Brian <[EMAIL PROTECTED]> <brian@[uk.]FreeBSD.org>
>
> Yup, Kirk committed it. I really like the changes -- in the old days
> disk caches were tiny and directories were not well cached on top of that.
> It made sense to try to keep directories close to their files.
>
> But today the proximity of a directory to its files is not really that
> important. It is far more important for directories to have reasonable
> proximity to each other not only to improve directory scans and lookups,
> but also to improve caching. Especially for small directories.
> Consider that small directories are typically contained in a single
> fragment (1K). If directories are spread all over the disk, caching
> is non-optimal. But if they are relatively close to each other then
> both our VM cache (if vfs.vmiodirenable is set to 1) and the hard
> drive's internal cache become extremely effective. With the added
Why VMIO dir works better if directories are placed close to each other? I
think it only makes the cache data of an individual directory stay in the
memory longer. Is there a way to measure the effectiveness of the disk
drive's cache?
-Zhihui
> effectiveness of the caches, seeking should wind up being
> significantly reduced even for things like 'tar'. Large directories
> also benefit, I think.
>
> From looking at the code, I don't think fragmentation will be an issue.
> Or, to be more specific, I don't think the fact that files may not wind
> up in the same cylinder group as their directory entry is an issue.
> Either you have a huge number of directories being accessed and need
> the locality of reference within the directory space even if it costs
> some additional seeking to access underlying files, or you don't and
> the active directories all wind up being cached, removing any additional
> seeking from the equation entirely.
>
> -Matt
>
>
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