Subject: Re: RFC: VOP_LOOKUP() speedup
To: Bill Studenmund <email@example.com>
From: Reinoud Zandijk <firstname.lastname@example.org>
Date: 05/06/2004 07:49:19
On Wed, May 05, 2004 at 09:12:28AM -0700, Bill Studenmund wrote:
> > So this is correct? i dont miss out a thing?
> With the second cache == block cache, yes, I think that's right.
Problem with UDF is that i can't use the device's block cache for CDs/DVDs
unless i could get it to use it only as a kind of read-`cache' that *never*
just writes back at its own will if i tell it so. To implement CD-RW and
DVD-RW, UDFs has to write with a fixed packet length; with CD-R and
DVD+R/DVD-R its obvious that i can only write in one `direction' i.e.
ordered; what happeneds with write-errors i dont even want to know ;) How
does the rest of the filingsystems deal with this situation?
Now it would propably advisable if i could claim/release the
`minicache'-lines dynamically from KVA; problem then is offcource still
when to release them... i dont want udfs to be a memory hogger ... the
current `minicache' is about 1Mb allready allthough it could be made
> I still don't understand why you want a number. Isn't a boolean (all here
> or not all here) enough?
In my tests with msdosfs i stumbled on the fact that maintaning the number
of entries in the directory wasn't that trivial esp. when ppl. start
> Perhaps I'm misunderstanding you, but my objection is to changing struct
> vnode or the name cache structures. I think all you need to do is either
> keep a flag or count in your fs's structure off the vnode.
This can work for sure with one guarantee: that i get a notification call
or a VOP_REVOKE after each namecache removal... thats the crux i think. If
the namecache is full, it starts deleting entries with its LRU algorithm
but doesn't notify the vnode nor the parent(s) vnode..... a new
> Also, if we find issues in the proposal that need tweaking, we can tweak
> things. ;-)
like this ;) ;)
> The problem is that using the namecache means having vnodes for all of
> these entities. So just doing an ls on a directory creates vnodes for all
> the contents.
true indeed.... that was my main objection too. There is one pitfall though
that might be applicable for non UDF too : when one does a `ls' and not a
readdir(), `ls' has to get file access and owner info, file type (softlink,
device, ...) and thus issues a VOP_LOOKUP() anyway and creates the vnodes
if you want or not :-/
> Given the costs of reading a directory for UDF, especially while writing,
> I think the cost of the extra vnodes is less than the cost of not having
> them. But for all other file systems, I do not expect the cost of the
> directory reads is worth the extra vnodes.
> The fact that we don't always create vnodes when we read a directory is a
> feature for a lot of workloads. I think we need to keep it (for everything
> other than UDF).
I came up with a less but efficient implementation: i can set a flag in a
directory inode that the inode has been created from scratch and not
read-in. A call to the still imaginary VOP_NAMECACHEOP() would then clear
this flag and thus retracts the autorisation of the namecache lookup.