Prefaulting for i/o buffers

[Attilio Rao]

2012/3/1, Pawel Jakub Dawidek <pjd at freebsd.org>:
> On Thu, Mar 01, 2012 at 01:16:24PM +0200, Gleb Kurtsou wrote:
>> On (25/02/2012 20:46), Pawel Jakub Dawidek wrote:
>> > - "Every file system needs cache. Let's make it general, so that all
>> > file
>> >   systems can use it!" Well, for VFS each file system is a separate
>> >   entity, which is not the case for ZFS. ZFS can cache one block only
>> >   once that is used by one file system, 10 clones and 100 snapshots,
>> >   which all are separate mount points from VFS perspective.
>> >   The same block would be cached 111 times by the buffer cache.
>>
>> Hmm. But this one is optional. Use vop_cachedlookup (or call
>> cache_entry() on your own), add a number of cache_prune calls. It's
>> pretty much library-like design you describe below.
>
> Yes, namecache is already library-like, but I was talking about the
> buffer cache. I managed to bypass it eventually with suggestions from
> ups@, but for a long time I was sure it isn't at all possible.

Can you please clarify on this as I really don't understand what you mean?

>
>> Everybody agrees that VFS needs more care. But there haven't been much
>> of concrete suggestions or at least there is no VFS TODO list.
>
> Everybody agrees on that, true, but we disagree on the direction we
> should move our VFS, ie. make it more light-weight vs. more heavy-weight.

All I'm saying (and Gleb too) is that I don't see any benefit in
replicating all the vnodes lifecycle at the inode level and in the
filesystem specific implementation.
I don't see a semplification in the work to do, I don't think this is
going to be simpler for a single specific filesystem (without
mentioning the legacy support, which means re-implement inode handling
for every filesystem we have now), we just loose generality.

if you want a good example of a VFS primitive that was really
UFS-centric and it was mistakenly made generic is vn_start_write() and
sibillings. I guess it was introduced just to cater UFS snapshot
creation and then it poisoned other consumers.

Thanks,
Attilio


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