Lustre discuss - May 2008 - Wide area use of Lustre and client caches

During the LUG I was approached by a customer who wants to use a Lustre file
system at the far end of a WAN link.  Since the situation may be of general
interest, I thought I would post a short report of the discussion here.

His use pattern was interesting ? a number of Windows clients must be
browsing files stored in Lustre in this remote location.  It was expected
that the files would be fairly large, would be viewed by multiple clients,
and that few or no modifications would be made.

After some discussion we proposed a solution that involved a deployment as
follows:

1. A single Lustre client with lots of RAM.  The settings on the client
would be (1) that the memory available for caching by lustre is large (2)
that the number of locks that can be held by this client is fairly large (3)
that this client uses the ?open cache?.
2. A samba server on this Lustre client.

With the settings above, we can expect that many of the files can be cached
in the Lustre client, hence after the initial read, I/O would be local in
the remote site.  With the open file cache enabled, even the open and close
traffic will not go to the servers, but can be handled by the client.   We
think that this will lead to a very good solution, that can work today.

A refinement is possible, that requires some development.  There is a
feature in the Linux kernel to use a disk partition as a cache for a file
system ? it is called cachefs.   This requires a few hooks in Lustre to
store chunks of files that are transferred to the client into this cache,
and cache invalidation calls to remove them.  It allows us to achieve the
same performance as with the solution above, except that the disk will be a
bit slower than memory, but it can also be much larger.

We are eagerly awaiting the results of testing this configuration!

- peter -
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On Thu, 2008-05-08 at 22:55 -0600, Peter Braam wrote:
> 
> His use pattern was interesting ? a number of Windows clients must be
> browsing files stored in Lustre in this remote location.  It was
> expected that the files would be fairly large, would be viewed by
> multiple clients, and that few or no modifications would be made.
Even still it''s useful during implementation to think of the use case
of
that remote client having read a file and caching and holding a read
lock on that file, say 1GB in size, and then another client wanting to
update say, 1KB in the middle of the file.  It would be beneficial for
that 1GB file to have a small (but still practical) stripe size so that
the amount of cache that needs to be thrown away to accommodate the
write is relatively small.

b.

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Nono - striping should only be used to get more bandwidth from servers.  The
correct solution to the problem you point out is a lock conversion, planned
long ago, still far away maybe (Nikita?).

Peter


On 5/9/08 8:25 AM, "Brian J. Murrell" <Brian.Murrell at Sun.COM>
wrote:
> On Thu, 2008-05-08 at 22:55 -0600, Peter Braam wrote:
>> 
>> His use pattern was interesting ? a number of Windows clients must be
>> browsing files stored in Lustre in this remote location.  It was
>> expected that the files would be fairly large, would be viewed by
>> multiple clients, and that few or no modifications would be made.
> 
> Even still it''s useful during implementation to think of the use
case of
> that remote client having read a file and caching and holding a read
> lock on that file, say 1GB in size, and then another client wanting to
> update say, 1KB in the middle of the file.  It would be beneficial for
> that 1GB file to have a small (but still practical) stripe size so that
> the amount of cache that needs to be thrown away to accommodate the
> write is relatively small.
> 
> b.
> 
> _______________________________________________
> Lustre-devel mailing list
> Lustre-devel at lists.lustre.org
> http://lists.lustre.org/mailman/listinfo/lustre-devel

On Fri, 2008-05-09 at 09:08 -0600, Peter Braam wrote:
> Nono - striping should only be used to get more bandwidth from servers. 
The
> correct solution to the problem you point out is a lock conversion,
Indeed, I have heard that term being used and figured that that is what
it was all about, however...
> planned
> long ago, still far away maybe (Nikita?).
Given that it''s not yet available and won''t be for some time,
isn''t
breaking a file up into many objects via striping a sufficient
alternative to the problem?

Are there other problems caused by striping a file (that technically
doesn''t need more bandwidth) that outweigh the benefits of not having
to
toss so much cache away when a file is written to (i.e. assuming the
cost of reading the file is high such as it would be on a WAN link)?

b.

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Peter Braam writes:
 > Nono - striping should only be used to get more bandwidth from servers. 
The
 > correct solution to the problem you point out is a lock conversion,
planned
 > long ago, still far away maybe (Nikita?).

Yes, it''s far away. Interestingly, similar lock "conversion"
for
meta-data locks is required for write-back cache, when a sub-tree lock
is split into a set of sub-sub-tree sub-locks.

 > 
 > Peter

Nikita.

On May 09, 2008  09:08 -0600, Peter J. Braam wrote:
> Nono - striping should only be used to get more bandwidth from servers. 
The
> correct solution to the problem you point out is a lock conversion, planned
> long ago, still far away maybe (Nikita?).
This task is something that Oleg has been working on occasionally,
I think there are patches around but fairly old (though not as bad
as might be expected, because LDLM code changes relatively slowly).
> On 5/9/08 8:25 AM, "Brian J. Murrell" <Brian.Murrell at
Sun.COM> wrote:
> > Even still it''s useful during implementation to think of the
use case of
> > that remote client having read a file and caching and holding a read
> > lock on that file, say 1GB in size, and then another client wanting to
> > update say, 1KB in the middle of the file.  It would be beneficial for
> > that 1GB file to have a small (but still practical) stripe size so
that
> > the amount of cache that needs to be thrown away to accommodate the
> > write is relatively small.
Having multiple stripes would save invalidation of (nstripes - 1) / nstripes
of the file, but in general the "update in the middle" paradigm is
very
rare in real life, so in practise I don''t think this will help much.

Even the "add a byte in the middle of a text file" case always causes
the whole file to be rewritten because of backing up the old file.

The only common applications I''m aware of that do partial-file
read/write
operations are databases and peer-to-peer file sharing.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.