linux-ext4 - Re: [PATCH 0/3] Batched discard support

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Message-ID: <alpine.LFD.2.00.1008101328400.2831@localhost>
Date:	Tue, 10 Aug 2010 13:31:55 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	jmoyer@...hat.com, rwheeler@...hat.com, eshishki@...hat.com,
	sandeen@...hat.com, jack@...e.cz, tytso@....edu
Subject: Re: [PATCH 0/3] Batched discard support

On Fri, 6 Aug 2010, Dmitry Monakhov wrote:

> Lukas Czerner <lczerner@...hat.com> writes:
> 
> > On Fri, 6 Aug 2010, Lukas Czerner wrote:
> >
> >> On Fri, 6 Aug 2010, Dmitry Monakhov wrote:
> >> 
> >> > Lukas Czerner <lczerner@...hat.com> writes:
> >> > 
> >> > > Hi, all
> >> > >
> >> > > because people were worried about possibly long stalls appearing
> >> > > when FITRIM ioctl is working, I have changed the FITRIM interface
> >> > > as Dimitry suggested. Now you can choose whether to trim whole
> >> > > file system or just a part of it, resp. you can specify the range
> >> > > of Bytes to trim.
> >> > Agree with whole patch-set, except minor note for ext4'th path.
> >> > Please feel free to add
> >> > Reviewed-by: Dmitry Monakhov <dmonakhov@...nvz.org> to the series
> >> > 
> >> > The only thing what is still not obvious for me is that, there are
> >> > several types of discard request possible
> >> > 1) Simple discard 
> >> > 2) Secure discard which was proposed here http://lkml.org/lkml/2010/6/24/71
> >> > Should we specify which type should be used in ioctl flags?
> >> > But i hope that we can just stick maximum security scenario
> >> > Use secure discard if possible.
> >> 
> >> First of all, thanks for you review Dimitry. And second, to be honest I
> >> am not entirely familiar with the Secure discard implementation. Right
> >> now it just doing the simple discard like "send TRIM command", so it
> >> does work just for devices which supports it. I suppose we can just
> >> check blk_queue_discard() at some level and then decide whether to do
> >> simple discard (TRIM), or secure discard "Write zeroes", when the device
> >> does not support TRIM - if it is what you mean by secure discard.
> >> 
> >> Regards
> >> -Lukas
> >> 
> >
> > When I am thinking about this, it may not be a bad idea to create a
> > completely new ioctl for this purpose of "zeroing all free space". We
> > do the trimming for completely different reasons, and the "secure"
> Actually you may be right here.
> For example it is usual to give some one an usb stick, 
> and always assumes what USB stick is WhatYouSeeIsWhatYouGet storage.
> but this is obviously not true, a man now has full access to that
> device, so  stale data is almost transparently available.
> Off course i can use SECRM but it has runtime overhead.
> So i can easily call SECDISCARD (even in emulation mode) before umount
> in order to be on safe side and then share my USB stick without any
> fears.
> 
> > thing is just an side effect, so we probably should not mix it together.
> >
> > The new ioclt (FISECER ?) and FITRIM can use the same infrastructure in
> > ext3/4, but we should add a flag to distinguish what we need to do -
> > TRIM or secure erase. What do you think ?
> Or we can just add a behavior flags filed
> DISCARD    :will works only for SDD and return ENOTSUPP for others
> SECURE_DEL :will guarantee that data will be zeroed on success.
> 
> DISCARD ->    (simple discard) send discard requests
> SECURE_DEL -> (simple emulation) write free space with zeroes
> (DISCARD|SECURE_DEL) -> send discards request with secure flag enabled.

Ok, so lets finish this simple discard thing first and then, when there
will be a "real" interest in this we can do something about it.

Thanks, Dimitry.
-Lukas

> >
> > Regards
> > -lukas
> > --
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