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From Daryn Sharp <da...@yahoo-inc.com>
Subject Re: fs cache giving me headaches
Date Tue, 07 Aug 2012 15:25:29 GMT
There is no UGI caching, so each request will receive a unique UGI even for the same user.
 Thus you can safely call FileSystem.closeAllForUGI(ugi) when the request is complete.  If
however you spin off threads that continue to use the UGI even after the request is completed,
then you'll have to determine for yourself when it's safe to close the filesystems.

I've been kicking around a few ways to transparently close cached filesystems for a ugi when
that ugi goes out of scope.  I should probably file a jira (if it stops going down) for discussion.


On Aug 7, 2012, at 10:15 AM, Koert Kuipers wrote:

The problem with FileSystem.closeAllForUGI(ugi) for me is that a server can be multi-threaded,
and a user could be doing multiple request at the same time, so if i used closeAllForUGI isn't
there a risk of shutting down the other requests for the same user?

On Mon, Aug 6, 2012 at 2:52 PM, Daryn Sharp <daryn@yahoo-inc.com<mailto:daryn@yahoo-inc.com>>
Yes, the implementation of fs.close() leaves something to be desired.  There's actually been
debate in the past about close being a no-op for a cached fs, but the idea was rejected by
the majority of people.

In the server case, you can use FileSystem.closeAllForUGI(ugi) at the end of a request to
flush all the fs cache entries for the ugi.  You'll get the benefit of the cache during execution
of the request, and be able to close the cached fs instances to prevent memory leaks. I hope
this helps!


On Aug 6, 2012, at 12:32 PM, Koert Kuipers wrote:

---------- Forwarded message ----------
From: "Koert Kuipers" <koert@tresata.com<mailto:koert@tresata.com>>
Date: Aug 4, 2012 1:54 PM
Subject: fs cache giving me headaches
To: <common-user@hadoop.apache.org<mailto:common-user@hadoop.apache.org>>

nothing has confused me as much in hadoop as FileSystem.close().
any decent java programmer that sees that an object implements Closable writes code like this:
Final FileSystem fs = FileSystem.get(conf);
try {
    // do something with fs
} finally {

so i started out using hadoop FileSystem like this, and i ran into all sorts of weird errors
where FileSystems in unrelated code (sometimes not even my code) started misbehaving and streams
where unexpectedly shut. Then i realized that FileSystem uses a cache and close() closes it
for everyone! Not pretty in my opinion, but i can live with it. So i checked other code and
found that basically nobody closes FileSystems. Apparently the expected way of using FileSystems
is to simple never close them. So i adopted this approach (which i think is really contrary
to java conventions for a Closeable).

Lately i started working on some code for a daemon/server where many FileSystems objects are
created for different users (UGIs) that use the service. As it turns out other projects have
run into trouble with the FileSystem cache in situations like this (for example, Scribe and
Hoop). I imagine the cache can get very large and cause problems (i have not tested this myself).

Looking at the code for Hoop i noticed they simply turned off the FileSystem cache and made
sure to close every FileSystem. So here the suggested approach to deal with FileSystems seems
to be:
Final FileSystem fs = FileSystem.newInstance(conf); // or FileSystem.get(conf) but with caching
turned off in the conf
try {
    // do something with fs
} finally {

This code bypasses the cache if i understand it correctly, avoiding any cache size limitations.
However if i adopt this approach i basically can not re-use any existing code or libraries
that do not close FileSystems, splitting the codebase into two which is pretty ugly. And this
code is not efficient in situations where there are very few used FileSystem objects and a
cache would improve performance, so the split works both ways.

In short, there is so single way to code with FileSystem that works in both situations! Ideally
i would have liked fs.close() to do the right thing depending in the settings: if cache is
turned off it closes the FileSystem, and if it is turned on its a NOOP. That way i could always
use FileSystem.get(conf) and always close my filesystems, and the code would be usable irrespective
of whether the cache is turned on or off.

Any insights or suggestions? Thanks!

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