Hi everybody,
I'm jumping in as Jeff is away due to an unexpected annoyance involving
Californian wildlife.
On 8/7/13 7:47 PM, Andrew Wang wrote:
Blocks are supposed to be an internal abstraction within HDFS, and aren't an
inherent part of FileSystem (the user-visible class used to access all Hadoop
filesystems).
Yes, but it's a really useful abstraction :) Do you really believe the blocks
could be abandoned in the next couple of years? I mean, it's such a simple and
effective solution ...
Is it possible to instead deal with files and offsets? On a read failure, you
could open a stream to the same file on the backup filesystem, seek to the old
file position, and retry the read. This feels like it's possible via wrapping.
As Jeff briefly mentioned, all USCMS sites export their data via XRootd (not all
of them use HDFS!) and we developed a specialization of XRootd caching proxy
that can fetch only requested blocks (block size is passed from our input stream
class to XRootd client (via JNI) and on to the proxy server) and keep them in a
local cache. This allows as to do three things:
1. the first time we notice a block is missing, a whole block is fetched from
elsewhere and further access requests from the same process get fulfilled with
zero latency;
2. later requests from other processes asking for this block are fulfilled
immediately (well, after the initial 3 retries);
3. we have a list of blocks that were fetched and we could (this is what we want
to look into in the near future) re-inject them into HDFS if the data loss turns
out to be permanent (bad disk vs. node that was offline/overloaded for a while).
Handling exceptions at the block level thus gives us just what we need. As input
stream is the place where these errors become known it is, I think, also the
easiest place to handle them.
I'll understand if you find opening-up of the interfaces in the central
repository unacceptable. We can always apply the patch at the OSG level where
rpms for all our deployments get built.
Thanks & Best regards,
Matevz
On Wed, Aug 7, 2013 at 3:29 PM, Jeff Dost <jd...@ucsd.edu
<mailto:jd...@ucsd.edu>> wrote:
Thank you for the suggestion, but we don't see how simply wrapping a
FileSystem object would be sufficient in our use case. The reason why is we
need to catch and handle read exceptions at the block level. There aren't
any public methods available in the high level FileSystem abstraction layer
that would give us the fine grained control we need at block level read
failures.
Perhaps if I outline the steps more clearly it will help explain what we are
trying to do. Without our enhancements, suppose a user opens a file stream
and starts reading the file from Hadoop. After some time, at some position
into the file, if there happen to be no replicas available for a particular
block for whatever reason, datanodes have gone down due to disk issues, etc.
the stream will throw an IOException (BlockMissingException or similar) and
the read will fail.
What we are doing is rather than letting the stream fail, we have another
stream queued up that knows how to fetch the blocks elsewhere outside of our
Hadoop cluster that couldn't be retrieved. So we need to be able to catch
the exception at this point, and these externally fetched bytes then get
read into the user supplied read buffer. Now Hadoop can proceed to read in
the stream the next blocks in the file.
So as you can see this method of fail over on demand allows an input stream
to keep reading data, without having to start it all over again if a failure
occurs (assuming the remote bytes were successfully fetched).
As a final note I would like to mention that we will be providing our
failover module to the Open Science Grid. Since we hope to provide this as
a benefit to all OSG users running at participating T2 computing clusters,
we will be committed to maintaining this software and any changes to Hadoop
needed to make it work. In other words we will be willing to maintain any
implementation changes that may become necessary as Hadoop internals change
in future releases.
Thanks,
Jeff
On 8/7/13 11:30 AM, Andrew Wang wrote:
I don't think exposing DFSClient and DistributedFileSystem members is
necessary to achieve what you're trying to do. We've got wrapper
FileSystems like FilterFileSystem and ViewFileSystem which you might be
able to use for inspiration, and the HCFS wiki lists some third-party
FileSystems that might also be helpful too.
On Wed, Aug 7, 2013 at 11:11 AM, Joe Bounour <jboun...@ddn.com
<mailto:jboun...@ddn.com>> wrote:
Hello Jeff
Is it something that could go under HCFS project?
http://wiki.apache.org/hadoop/__HCFS
<http://wiki.apache.org/hadoop/HCFS>
(I might be wrong?)
Joe
On 8/7/13 10:59 AM, "Jeff Dost" <jd...@ucsd.edu
<mailto:jd...@ucsd.edu>> wrote:
Hello,
We work in a software development team at the UCSD CMS Tier2
Center. We
would like to propose a mechanism to allow one to subclass the
DFSInputStream in a clean way from an external package. First
I'd like
to give some motivation on why and then will proceed with the
details.
We have a 3 Petabyte Hadoop cluster we maintain for the LHC
experiment
at CERN. There are other T2 centers worldwide that contain
mirrors of
the same data we host. We are working on an extension to Hadoop
that,
on reading a file, if it is found that there are no available
replicas
of a block, we use an external interface to retrieve this block
of the
file from another data center. The external interface is
necessary
because not all T2 centers involved in CMS are running a Hadoop
cluster
as their storage backend.
In order to implement this functionality, we need to subclass
the
DFSInputStream and override the read method, so we can catch
IOExceptions that occur on client reads at the block level.
The basic steps required:
1. Invent a new URI scheme for the customized "FileSystem" in
core-site.xml:
<property>
<name>fs.foofs.impl</name>
<value>my.package.__FooFileSystem</value>
<description>My Extended FileSystem for foofs:
uris.</description>
</property>
2. Write new classes included in the external package that
subclass the
following:
FooFileSystem subclasses DistributedFileSystem
FooFSClient subclasses DFSClient
FooFSInputStream subclasses DFSInputStream
Now any client commands that explicitly use the foofs:// scheme
in paths
to access the hadoop cluster can open files with a customized
InputStream that extends functionality of the default hadoop
client
DFSInputStream. In order to make this happen for our use case,
we had
to change some access modifiers in the DistributedFileSystem,
DFSClient,
and DFSInputStream classes provided by Hadoop. In addition, we
had to
comment out the check in the namenode code that only allows for
URI
schemes of the form "hdfs://".
Attached is a patch file we apply to hadoop. Note that we
derived this
patch by modding the Cloudera release hadoop-2.0.0-cdh4.1.1
which can be
found at:
http://archive.cloudera.com/__cdh4/cdh/4/hadoop-2.0.0-cdh4.__1.1.tar.gz
<http://archive.cloudera.com/cdh4/cdh/4/hadoop-2.0.0-cdh4.1.1.tar.gz>
We would greatly appreciate any advise on whether or not this
approach
sounds reasonable, and if you would consider accepting these
modifications into the official Hadoop code base.
Thank you,
Jeff, Alja & Matevz
UCSD Physics
