Re: O_DIRECT question

[Denis Vlasenko]

On Monday 22 January 2007 17:17, Phillip Susi wrote:
> > You do not need to know which read() exactly failed due to bad disk.
> > Filename and offset from the start is enough. Right?
> > 
> > So, SIGIO/SIGBUS can provide that, and if your handler is of
> > 	void (*sa_sigaction)(int, siginfo_t *, void *);
> > style, you can get fd, memory address of the fault, etc.
> > Probably kernel can even pass file offset somewhere in siginfo_t...
> 
> Sure... now what does your signal handler have to do in order to handle 
> this error in such a way as to allow the one request to be failed and 
> the task to continue handling other requests?  I don't think this is 
> even possible, yet alone clean.

Actually, you have convinced me on this. While it's is possible
to report error to userspace, it will be highly nontrivial (read:
bug-prone) for userspace to catch and act on the errors.

> > You think "Oracle". But this application may very well be
> > not Oracle, but diff, or dd, or KMail. I don't want to care.
> > I want all big writes to be efficient, not just those done by Oracle.
> > *Including* single threaded ones.
> 
> Then redesign those applications to use aio and O_DIRECT.  Incidentally 
> I have hacked up dd to do just that and have some very nice performance 
> numbers as a result.

I will still disagree on this point (on point "use O_DIRECT, it's faster").
There is no reason why O_DIRECT should be faster than "normal" read/write
to large, aligned buffer. If O_DIRECT is faster on today's kernel,
then Linux' read()/write() can be optimized more.

(I hoped that they can be made even *faster* than O_DIRECT, but as I said,
you convinced me with your "error reporting" argument that reads must still
block until entire buffer is read. Writes can avoid that - apps can do
fdatasync/whatever to make sync writes & error checks if they want).
--
vda
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