On Mon, Dec 29, 2008 at 11:50:14AM +0530, [email protected] wrote:
> Andreas Dilger wrote:
> > On Dec 25, 2008 12:35 -0200, Alberto Bertogli wrote:
> > > On Wed, Dec 24, 2008 at 06:49:15PM -0500, Theodore Tso wrote:
> > > > I have no idea why you would need to do manual __breads(). No
> doubt
> > > > I'm missing some context here.
> > >
> > > I'm writing (just for fun and learning purposes) a device mapper
> > > module that stores checksums on writes and verifies them on reads.
> The
> > > integrity metadata (currently just the checksum) is interleaved in
> the
> > > backing device: one sector holding the integrity metadata for the
> > > following 64 data sectors.
>
> > Alex and I discussed implementing checksums for ext4 using an external
> device like this, and he might have some more design information for
> you.
>
>
> That external device could possibly be a TPM chip that can store
> checksums.
Thanks for the suggestion. The code I have at the moment (without the
journal stuff) already has the capability of storing checksums in a
different device. It's one of the reasons why I would prefer to avoid
using jbd.
I think I'll go with the "two metadatas" approach and see how it goes.
Worst case scenario is that I have to drop that code, which means to be
back where I am now, only with one less option.
Thanks,
Alberto
one good thing about JBD is that you can't update target block and csum
atomically. so, either you use some form of COW or you use journalling.
given we already have JBD it'd make sense to use it?
thanks, Alex
Alberto Bertogli wrote:
> I think I'll go with the "two metadatas" approach and see how it goes.
> Worst case scenario is that I have to drop that code, which means to be
> back where I am now, only with one less option.
>
> Thanks,
> Alberto
>
On Tue, Dec 30, 2008 at 09:55:57AM +0300, Alex Tomas wrote:
> one good thing about JBD is that you can't update target block and csum
> atomically. so, either you use some form of COW or you use journalling.
> given we already have JBD it'd make sense to use it?
I'm sorry, but I'm not following. Is that first sentence right?
The main disadvantage I see of using jbd at the moment is that I loose
the possibility of having checksums and data in a different device.
The only alternative to jbd that I have at the moment is the "two
metadatas" approach I explained in another email (but please let me know
if it wasn't clear).
They both provide what I need (atomicity in data and csum writes), one
is easier, more tested, but prevents a feature. The other is a bit more
difficult, untested and written my me, but allows a feature. I have no
idea, performance-wise, how they will behave (it is expected they suck,
according to the other emails).
At this moment I'm going with the two metadatas approach, because I
think it has less limitations and it'd be fun to write. If then it's
unfit for some reason, I can always go back and use jbd. But I'm
obviously open to suggestions and more alternatives.
Thanks a lot,
Alberto