Hi Chris,
On 10/03/2016 01:21 AM, Chris wrote:
Hello Clay,
Thanks for your answer it helped me to understand the challenge around
this use case. Actually I thought it would not be straight forward
because as you mentioned swift is driven by the partitions.
Anyhow we are about to provide an object store service powered by
swift. The users see this service from their file perspective, so the
questions I get ask are more likely like:
·Does my file has all the necessary replications
·If my file isn’t fully replicated yet what queue has it in the backlog
·How long will it take until my file has all the necessary replicas
The user will accept that swift is not a real time replication and
it’s based on the concept of eventual consistency, but does this
exclude visibility about the internal processes and metrics.
I *think* what you are getting at is a misconception about how Swift and
eventual consistency works. I've heard that before where people think in
the case of swift, eventual consistency means that when a user sends a
PUT request to swift, *one* copy is written to disk and that there's a
background process replicating the object to make it durable. This is
not the case...
Swift will only return a success message to the user, once the object
has been durably written to disk. Durably means at least a quorum has
been reached. In the case of 3x replication, a user will receive a
success message (HTTP 201 Created) when at least 2 copies of the object
has been written (and fsynced) to disk.
When the swift proxy is unable to write data to a primary device, it
will write the data to a hand-off device. In this case, the background
replication process will attempt to move that object to the primary
device once the problem is fixed. During failure scenarios like this, it
is possible to read stale data, but Swift will eventually move data to
primary nodes so that the correct, most current data is always read.
I hope this helps, the docs [1] also has a ton of useful info...
Regards,
Thiago
[1] - http://docs.openstack.org/developer/swift/overview_architecture.html
Cheers
Chris
*From:*Clay Gerrard [mailto:[email protected]]
*Sent:* Saturday, October 01, 2016 01:16
*To:* Chris <[email protected]>
*Cc:* Openstack <[email protected]>
*Subject:* Re: [Openstack] Swift replication backlog and replication
status for specific file
To look at current availability of a single object you can use
`swift-get-nodes` and check all of the primary locations - or if you
have the `.data` file handy already you can use `swift-object-info`
Either of these options will tell you were the object should be, and
also where it might be if there was a failure leading to handoff.
There's even some output to help you spot check those locations.
OTOH, if you have recently rebalanced you can use either of these
tools with the old ring as well.
... but honestly unless your troubleshooting a specific failure these
things may not work exactly according to your requirements. They
aren't really designed to be consumed on an "object-by-object" basis.
There's just too many objects!
As you pointed out swift tends to break things down by partition - and
there's already a lot of partition-replicas! (3 replicas * 2^16
partitions is a lot!)
One of the more recent ideas about how to visualize replication
backlog is too look at all the partitions acctually on disk and see
how it differs from the partitions that are assigned in the ring -
some of the idea is fleshed out here:
http://docs.openstack.org/developer/swift/admin_guide.html#checking-handoff-partition-distribution
Maybe you have some different ideas? Or a different use-case? Or
maybe it's a common goal that lots of deployments are solving in
various ways or maybe something no one really currently has a good
handle on?
Are you working a solution to a particular problem in your deployment
or anticipating a need down the road?
-Clay
On Fri, Sep 30, 2016 at 5:01 AM, Chris <[email protected]
<mailto:[email protected]>> wrote:
Hello,
How is it possible to get the following information from swift:
-Backlog of all replications based on files
-Query the replication status for a specific file
I know there is the swift-dispersion-report but this gives the
partition view. We are more interested in the file view
Thanks in advance.
Cheers,
Chris
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