openstack team mailing list archive

[Stephen Broeker <sbroeker@xxxxxxxxxxxx>]

If a node is down, then it is ignored.
That is the whole point about 3 replicas.

On Fri, Jan 20, 2012 at 10:43 AM, Nikolaus Rath <Nikolaus@xxxxxxxx> wrote:

> Hi,
>
> What happens if one of the nodes is down? Especially if that node holds
> the newest copy?
>
> Thanks,
> Nikolaus
>
> On 01/20/2012 12:33 PM, Stephen Broeker wrote:
> > The X-Newest header can be used by a GET Operation to ensure that all of
> the
> > Storage Nodes (3 by default) are queried for the latest copy of the
> Object.
> > The COPY Object operation already has this functionality.
> >
> > On Fri, Jan 20, 2012 at 9:12 AM, Nikolaus Rath <Nikolaus@xxxxxxxx
> > <mailto:Nikolaus@xxxxxxxx>> wrote:
> >
> >     Hi,
> >
> >     No one able to further clarify this?
> >
> >     Does swift offer there read-after-create consistence like
> >     non-us-standard S3? What are the precise syntax and semantics of
> >     X-Newest header?
> >
> >     Best,
> >     Nikolaus
> >
> >
> >     On 01/18/2012 10:15 AM, Nikolaus Rath wrote:
> >     > Michael Barton <mike-launchpad@xxxxxxxxxxxxxxxx
> >     <mailto:mike-launchpad@xxxxxxxxxxxxxxxx>> writes:
> >     >> On Tue, Jan 17, 2012 at 4:55 PM, Nikolaus Rath <Nikolaus@xxxxxxxx
> >     <mailto:Nikolaus@xxxxxxxx>> wrote:
> >     >>> Amazon S3 and Google Storage make very explicit (non-)
> consistency
> >     >>> guarantees for stored objects. I'm looking for a similar
> >     documentation
> >     >>> about OpenStack's Swift, but haven't had much success.
> >     >>
> >     >> I don't think there's any documentation on this, but it would
> >     probably
> >     >> be good to write up.  Consistency in Swift is very similar to S3.
> >     >> That is, there aren't many non-eventual consistency guarantees.
> >     >>
> >     >> Listing updates can happen asynchronously (especially under
> >     load), and
> >     >> older versions of files can show up in requests (deletes are just
> a
> >     >> new "deleted" version of the file).
> >     >
> >     > Ah, ok. Thanks a lot for stating this so explicitly. There seems
> >     to be a
> >     > lot of confusion about this, now I can at least point people to
> >     > something.
> >     >
> >     >> Swift can generally be relied on for read-after-write consistency,
> >     >> like S3's regions other than the the US Standard region.  The
> reason
> >     >> S3 in US Standard doesn't have this guarantee is because it's more
> >     >> geographically widespread - something Swift isn't good at yet.  I
> can
> >     >> imagine we'll have the same limitation when we get there.
> >     >
> >     > Do you mean read-after-create consistency? Because below you say
> about
> >     > read-after-write:
> >     >
> >     >>> - If I receive a (non-error) response to a PUT request, am I
> >     guaranteed
> >     >>> that the object will be immediately included in all object
> >     listings in
> >     >>> every possible situation?
> >     >>
> >     >> Nope.
> >     >
> >     > ..so is there such a guarantee for PUTs of *new* objects (like S3
> non
> >     > us-classic), or does "can generally be relied on" just mean that
> the
> >     > chances for new puts are better?
> >     >
> >     >> Also like S3, Swift can't make any strong guarantees about
> >     >> read-after-update or read-after-delete consistency.  We do have an
> >     >> "X-Newest" header that can be added to GETs and HEADs to make the
> >     >> proxy do a quorum of backend servers and return the newest
> available
> >     >> version, which greatly improves these, at the cost of latency.
> >     >
> >     > That sounds very interesting. Could you give some more details on
> what
> >     > exactly is guaranteed when using this header? What happens if the
> >     server
> >     > having the newest copy is down?
> >     >
> >     >>> - If the swift server looses an object, will the object name
> >     still be
> >     >>> returned in object listings? Will attempts to retrieve it result
> >     in 404
> >     >>> errors (as if it never existed) or a different error?
> >     >>
> >     >> It will show up in listings, but give a 404 when you attempt to
> >     >> retrieve it.  I'm not sure how we can improve that with Swift's
> >     >> general model, but feel free to make suggestions.
> >     >
> >     > From an application programmers point of view, it would be very
> >     helpful
> >     > if lost objects could be distinguished from non-existing object by
> a
> >     > different HTTP error. Trying to access a non-existing object may
> >     > indicate a bug in the application, so it would be nice to know
> when it
> >     > happens.
> >     >
> >     > Also, it would be very helpful if there was a way to list all lost
> >     > objects without having to issue HEAD requests for every stored
> object.
> >     > Could this information be added to the XML and JSON output of
> >     container
> >     > listings? Then an application would have the chance to periodically
> >     > check for lost data, rather than having to handle all lost objects
> at
> >     > the instant they're required.
> >     >
> >     >
> >     > I am working on a swift backend for S3QL
> >     > (http://code.google.com/p/s3ql/), a program that exposes online
> cloud
> >     > storage as a local UNIX file system. To prevent data corruption,
> there
> >     > are two requirements that I'm currently struggling to provide with
> the
> >     > swift backend:
> >     >
> >     > - There needs to be a way to reliably check if one object (holding
> the
> >     >   file system metadata) is the newest version.
> >     >
> >     >   The S3 backend does this by requiring storage in the non
> us-classic
> >     >   regions and using list-after-create consistency with a marker
> object
> >     >   that has has a "generation number" of the metadata embedded in
> its
> >     >   name.
> >     >
> >     >   I'm not yet sure if this would work with swift as well (the
> google
> >     >   storage backend just relies on the strong read-after-write
> >     >   consistency).
> >     >
> >     > - The file system checker needs a way to identify lost objects.
> >     >
> >     >   Here the S3 backend just relies on the durability guarantee that
> >     >   effectively no object will ever be lost.
> >     >
> >     >   Again, I'm not sure how to implement this for swift.
> >     >
> >     >
> >     > Any suggestions?
> >     >
> >     >
> >     >
> >     > Best,
> >     >
> >     >    -Nikolaus
> >     >
> >
> >
> >       -Nikolaus
> >
> >     --
> >      »Time flies like an arrow, fruit flies like a Banana.«
> >
> >      PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6  02CF A9AD B7F8 AE4E 425C
> >
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> >
> >
>
>
>   -Nikolaus
>
> --
>  »Time flies like an arrow, fruit flies like a Banana.«
>
>  PGP fingerprint: 5B93 61F8 4EA2 E279 ABF6  02CF A9AD B7F8 AE4E 425C
>