[Gluster-devel] Bitrot/Tering : Bad files get migrated and hence corruption goes undetected.

[Venky Shankar]

On Wed, Mar 02, 2016 at 11:43:50AM +0530, Venky Shankar wrote:
> On Sat, Feb 27, 2016 at 06:32:38AM -0500, Joseph Fernandes wrote:
> > Yep Agree! :)
> > 
> > Lets hear from the bitrot folks, what they have to propose.
> 
> Apologies for late reply.
> 
> > 
> > ~Joe 
> > 
> > ----- Original Message -----
> > From: "Niels de Vos" <ndevos at redhat.com>
> > To: "Joseph Fernandes" <josferna at redhat.com>
> > Cc: "Gluster Devel" <gluster-devel at gluster.org>
> > Sent: Saturday, February 27, 2016 4:28:43 PM
> > Subject: Re: [Gluster-devel] Bitrot/Tering : Bad files get migrated and hence corruption goes undetected.
> > 
> > On Fri, Feb 26, 2016 at 11:01:28PM -0500, Joseph Fernandes wrote:
> > > Well correctly we dont migrate the existing signature, the file starts
> > > it life fresh in the new tier(i.e get the bit rot version 1 on the new
> > > tier),
> > > Now this is also the case with any special xattr/attributes of the
> > > file.
> > > Again we rely heavily on the dht rebalance mechanism for migrations,
> > > which also doesnt carry special attributes/xattr.
> > 
> > Is there a good reason to not migrate the bitrot signature? Relying on
> > an existing functionality is fine, but if it does not address all your
> > needs, you have a valid use-case to improve it.
> 
> That could be done. However, an I/O operation on the object during migration
> should invalidate the signature and the object should be signed again.
> 
> AFAICS, there needs to be some infrastructure to avoid (re)signing of an
> object if it's fresh after migration.

[Replying to my own mail]

What's needed here is an hint that one or more open()'s happened on the
object in the migration window. In that case, the object version needs
to be incremented followed by a (re)sign after last close.

The other part that needs thinking is to scrub the object upon access
(if it's atleast signed).

> 
> Thoughts?
> 
> > 
> > Niels
> > 
> > > 
> > > 
> > > ----- Original Message -----
> > > From: "Niels de Vos" <ndevos at redhat.com>
> > > To: "Joseph Fernandes" <josferna at redhat.com>
> > > Cc: "Gluster Devel" <gluster-devel at gluster.org>
> > > Sent: Friday, February 26, 2016 10:33:11 PM
> > > Subject: Re: [Gluster-devel] Bitrot/Tering : Bad files get migrated and hence corruption goes undetected.
> > > 
> > > On Fri, Feb 26, 2016 at 09:32:46AM -0500, Joseph Fernandes wrote:
> > > > Hi All,
> > > > 
> > > > This is a discussion mail on the following issue, 
> > > > 
> > > > 1. Object is corrupted before it could be signed: In this case, the corrupted
> > > >    object is signed and get migrated upon I/O. There's no way to identify corruption
> > > >    for this set of objects.
> > > > 
> > > > 2. Object is signed (but not scrubbed) and corruption happens thereafter:
> > > >    In this case, as of now, integrity checking is not done on the fly
> > > >    and the object would get migrated (and signed again in the hot tier).
> > > > 
> > > > 
> > > > The (1) is definitely not a issue with bitrot with tiering. But (2) we can do something to avoid
> > > > corrupted file from getting migrated. Before we migrate files we can scrub it, but its just a naive
> > > > thought, any better suggestions? 
> > > 
> > > Is there a reason the existing signature can not be migrated? Why does
> > > it become invalid?
> > > 
> > > Niels
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