QFS qwrite/mh_write? What for?

[Ernest Siu]

I'm reading the QFS documentations and there're couple of functions
namely Qwrite and mh_write that allows multiple writers to perform
direct I/O onto the same file... What application can possibly make
use of this feature (safely without corrupting data)?

Database has multiple clients but it does not need this function
because the engine itself manages the queues and threads.

Regards,
Ernest

[Dragan Cvetkovic]

[email]ernestsiu@yahoo.com[/email] (Ernest Siu) writes:

> I'm reading the QFS documentations and there're couple of functions
> namely Qwrite and mh_write that allows multiple writers to perform
> direct I/O onto the same file... What application can possibly make
> use of this feature (safely without corrupting data)?

Well, the idea is that the responsibility for locking the data etc is on
you and your application instead of on OS (as it is the case in most other
file systems). I think that Oracle can use that feature.

In other words, in the case of QFS, you get a rope, but you can hang
yourself easily if you are not careful. In the case of UFS, you don't get a
rope at all.

Bye, Dragan

--
Dragan Cvetkovic,

To be or not to be is true. G. Boole No it isn't. L. E. J. Brouwer

!!! Sender/From address is bogus. Use reply-to one !!!

[Anton Rang]

[email]ernestsiu@yahoo.com[/email] (Ernest Siu) writes:

> I'm reading the QFS documentations and there're couple of functions
> namely Qwrite and mh_write that allows multiple writers to perform
> direct I/O onto the same file... What application can possibly make
> use of this feature (safely without corrupting data)?

Applications which perform their own file locking or coordination.
For instance, a distributed application may write in parallel to a
large output file, with each host writing only to one region of the
file. Or a database may use its own distributed lock manager to
ensure that a particular page of the database is only written by one
process (for qwrite) or host (for mh_write) at a time.

> Database has multiple clients but it does not need this function
> because the engine itself manages the queues and threads.

Actually, databases do benefit from this feature, as it gives them
increased concurrency. qwrite can be compared to veritas' quick i/o.

-- Anton