What's the "prepare_discard_fn" supposed to do?

What's the "prepare_discard_fn" supposed to do?

[Chris Worley]

In perusing the "discard-2.6" kernel at infradead.org (great job... I
have a device I'm testing it on, and it's working), I'm unclear on
what I'm supposed to do in the prepare_discard_fn function.

It looks like, if it exists, the the discard bio gets passed to the
normal handler.  Otherwise, it seems to have no duties.  I'm guessing
it's for whatever initialization is needed for the device...  but
checking to be sure.

Thanks,

Chris

Re: What's the "prepare_discard_fn" supposed to do?

[David Woodhouse]

On Thu, 2008-10-02 at 07:45 -0600, Chris Worley wrote:
> In perusing the "discard-2.6" kernel at infradead.org (great job... I
> have a device I'm testing it on, and it's working), I'm unclear on
> what I'm supposed to do in the prepare_discard_fn function.
> 
> It looks like, if it exists, the the discard bio gets passed to the
> normal handler.  Otherwise, it seems to have no duties.  I'm guessing
> it's for whatever initialization is needed for the device...  but
> checking to be sure.

It's intended to prepare the command which will be sent to the device,
much like the prepare_flush_fn does. See sd_prepare_flush() in
drivers/scsi/sd.c for an example (all the devices we've hooked up for
discard in Linux so far are virtual ones, so they don't make good
examples). I do have a real ATA drive with TRIM protocol support on its
way to me though, so I'll be looking at that some time soon.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation

Re: What's the "prepare_discard_fn" supposed to do?

[Artem Bityutskiy]

Hi David,

David Woodhouse wrote:
> It's intended to prepare the command which will be sent to the device,
> much like the prepare_flush_fn does. See sd_prepare_flush() in
> drivers/scsi/sd.c for an example (all the devices we've hooked up for
> discard in Linux so far are virtual ones, so they don't make good
> examples). I do have a real ATA drive with TRIM protocol support on its
> way to me though, so I'll be looking at that some time soon.

Sorry for my ignorance, but why "discard" is used in hard drive? What
may it improve? I've always been thinking it is only useful for SSD and
other FTL-enabled beasts.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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Re: What's the "prepare_discard_fn" supposed to do?

[David Woodhouse]

On Fri, 2008-10-03 at 09:48 +0300, Artem Bityutskiy wrote:
> Hi David,
> 
> David Woodhouse wrote:
> > It's intended to prepare the command which will be sent to the device,
> > much like the prepare_flush_fn does. See sd_prepare_flush() in
> > drivers/scsi/sd.c for an example (all the devices we've hooked up for
> > discard in Linux so far are virtual ones, so they don't make good
> > examples). I do have a real ATA drive with TRIM protocol support on its
> > way to me though, so I'll be looking at that some time soon.
> 
> Sorry for my ignorance, but why "discard" is used in hard drive? What
> may it improve? I've always been thinking it is only useful for SSD and
> other FTL-enabled beasts.

I said 'real ATA drive'. I didn't say it was magnetic.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation

Re: What's the "prepare_discard_fn" supposed to do?

[Matthew Wilcox]

On Fri, Oct 03, 2008 at 09:48:39AM +0300, Artem Bityutskiy wrote:
> Sorry for my ignorance, but why "discard" is used in hard drive? What
> may it improve? I've always been thinking it is only useful for SSD and
> other FTL-enabled beasts.

If you look at the work being done in T10, it's also envisaged as being
useful for large storage arrays where you do "thin provisioning" --
eg pretend that twenty clients each have a terabyte of disc and only
buy five terabytes of disc for the array.  When they start to use four
terabytes, you'll buy some more discs for it, and nobody will ever know.

In order to make this work well, you have to know when files really have
been deleted.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."