On Apr 20, 2007, at 10:47 AM, Anton B. Rang wrote:
ZFS uses caching heavily as well; much more so, in fact, than UFS.
Copy-on-write and direct i/o are not related. As you say, data gets
written first, then the metadata which points to it, but this isn't
anything like direct I/O. In particular, direct I/O avoids caching
the data, instead transferring it directly to/from user buffers,
while ZFS-style copy-on-write caches all data. ZFS does not have
direct I/O at all right now.
You're context is correct, but i'd be careful with "direct I/O", as i
think its an overloaded term that most people don't understand what
it does - just that it got them good performance (somehow). Roch has
a blog on this:
http://blogs.sun.com/roch/entry/zfs_and_directio
But you are correct that ZFS does not have the ability for the user
to say "don't cache user data for this filesystem" (which is one part
of direct I/O).
I've talked to some database people and they aren't convinced having
this feature would be a win. So if someone has a real world workload
where having the ability to purposely not cache user data would be a
win, please let me know.
eric
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