Scalable NFS Powered By Open Source Cluster Filesystems

40 Terabytes Per Week With Linux-based Clusters at Dunnhumby
It seems reasonable to think that this company tested the open source clustering stuff, but I don’t know for certain. There are folks out there using Open Source cluster filesystems for “large I/O” processing as is apparent in this recent OCFS2 bug report (emphasis added by me):

During maintenance window, decided to use the OCFS2 filesystem to store a large backup file (about 5-10 gig file). SCP’ed the file from an outside server to node1 of the cluster […]

A little third-party perspective is necessary. Not even back in 1990, with Fujitsu Swallow IV drives, was 10GB considered “large.” The OCFS2 user that filed the bug continued:

After a few minutes, node1 crashed.

Let’s think about that for a moment. The user is bringing unstructured data into the OCFS2 cluster filesystem using scp (1). Just for the heck of it, let’s take the user at his word and do the math. He said, “After a few minutes.” Let’s say a few minutes are 3—180 seconds. That means the scp(1) was likely not trafficked over Gigabit Ethernet because that would be more like enough time to move about 20GB at full bandwidth with a single wire. That pretty much leaves 100BaseT. So, somewhere along 2GB or so, OCFS2 crumbled. Hmmm, lowered expectations. And the fun continued:

Node1 restarted, but crashed again attempting to reenter the cluster.
Leaving Node1 down, attempted reboot of Node2 and Node3.
Both panic crashed during restart attempting to start OCFS2 and join the cluster.
Eventually, found that we had to start Node1 first, then restart the other two nodes.

Good grief, I’m not even going to comment on that bit, but I will point out that the suggested workaround to use the O_DIRECT enabled coreutils seems off mark. The user is trying to scp(1), not cp(1) or mv(1).

If It Isn’t Free, It’s Junk. Ad Revenue Funds Robust Software Development.
In spite of the fact that Ray Lane says traditional software products are soon to be replaced by cobbled together bits and pieces of open source stuff or what Wharton refers to as “ad supported software”, sometimes the good things in life are not free.

Huge Amounts of Unstructured Data
A recent article in Information Week’s Optimize Magazine covered one of PolyServe’s customers, Dunnhumby. These folks manipulate a lot of data using HP Blades as compute nodes accessing data over NFS in a PolyServe File Serving Utility scalable NAS solution. In their own words:

Each week, more than 40 terabytes of data is generated […]

“Hold it”, you say, that’s a comparison of OCFS2 to PolyServe CFS via NFS. What does OCFS2 have to do with NFS? That is a good question. OCFS2 is proclaimed to be a general purpose filesystem (emphasis added by me):

WHAT IS OCFS2?

OCFS2 is the next generation of the Oracle Cluster File System for Linux. It is an extent based, POSIX compliant file system. Unlike the previous release (OCFS), OCFS2 is a general-purpose file system

So why not export OCFS2 filesystems via NFS? That is the sort of thing you do with a general purpose filesystem after all. And, since OCFS2 is a cluster filesystem there shouldn’t be any second thoughts about exporting the same filesystems from multiple nodes—that’s scalable file serving. In fact, that has been tried before. That URL points to a bug report where a user was trying to implement scalable file serving using OCFS2. He reports:

I’m using OCSF2 for backups and to store files used by nfs clients. We have some errors during three file uploading from remote clients. In that case only one node can access those files but the other node receive from dlm a bad lockres error message […]

Right, OK. So what came next? Read on:

So I tried to stop ocfs2 and o2cb services on the second node but I can’t because heartbeat prevents any stop attempt. A stop attempt on the first node instead hungs and I have to reboot the first node because it is impossible to unmount ocfs2 filesystems (even if I use the lazy option).

I’m sure it couldn’t get any worse, right? He continued:

That is a serious problem because to recover the right functionality I had to reboot the first node (o2cb/ocfs2 services hang and after reboot ASM losts spfiles, so problem impacts even the databases running on cluster). There is any kind of action I can do to avoid that?

Surely he must be doing something really convoluted to hit problems so easily! He explains the scenario:

The scenario is:
node X exports filesystem to host Y
node W exports filesystem to host Z

from Y I create a file then I delete it then ls command on Z lists the file but I cannot open it. I receive I lot of messages like this:

Oct 20 08:53:34 proxb31 kernel: (15612,1):ocfs2_populate_inode:234 ERROR:
Invalid dinode: i_ino=9977187, i_blkno=9977187, signature = INODE01, flags = 0x0
Oct 20 08:53:34 proxb31 kernel: (15612,1):ocfs2_read_locked_inode:389 ERROR:
populate inode failed! i_blkno=9977187, i_ino=9977187

Good grief! Cache coherency problems? You mean like this warning about OCFS cache coherency :

Reasons for using odirect cp:

1. Buffered and direct ios are still racy in the kernel. As Oracle is doing directio, doing a normal cp exposes one to the chance of copying a stale page data.

2. Direct ios are less stressful on the page cache. As Oracle datafiles are invariably large, directio is more efficient in the long run.

3. In a clustered environment, the blocks on disk could be updated by any nodes in the cluster. Using odirect io ensures the latest version of the block is always read.

Oh boy. Anyway, back to the bug report. The bug report states that as of January 4, 2007, there is a patch for NFS exported OCFS2 problem being tested at Oracle, however, the following comment was given to help set expectations:

One thing I’m concerned with is having two clients connect to seperate nodes. Since NFSD is not cluster aware, there may be some issues with unlinked inodes being in cache on one node and looked up on another. Is it possibleto confine your nfs exports to a single node for now, until we can get a better handle on that particular issue.

That seems like something that should have been spelled out in the Product Requirements Document, but I’m old-fashioned.

Scalable File Serving with Linux. Who Needs a Cluster-Aware NFSD?
The NAS heads in a PolyServe File Serving Utility configuration (e.g., HP EFS Clustered Gateway), run the enterprise distributions; RHEL4 and SuSE SLES9. So while those folks in the Ray Lane and Wharton’s open source dream world might think that NFSD cannot function in a cluster with data consistency, PolyServe—with that dying traditional software model—seems to have pulled it off. Do you think Dunnhumby pushes 40TB of data per week through a PolyServe File Serving Utility cluster without NFSD scalability or—more importantly—cache coherency? Not a chance.



							

4 Responses to “Scalable NFS Powered By Open Source Cluster Filesystems”


  1. 1 ocfs2 user February 11, 2007 at 9:16 am

    Is Polyserve’s bug database open for public scrutiny?

  2. 2 kevinclosson February 11, 2007 at 5:18 pm

    ocfs2 user,

    That’s a fair question and I was wondering when it would be asked. No it is not open for scrutiny and we certainly do have bugs. In general where we differ from stuff like OCFS2 is that our bugs do not represent complete breaches of contract with our advertised features. That is, the bugs I pick out of ocfs2 when making a point are not the normal bugs one expects in all software. I pull out bugs that show OCFS2 doesn’t do what it says it does from a feature standpoint–most particularly in the realm of stability and general purpose application. In the end, PolyServe bugs are not manifestations of completely inadequate implementation decisions and architectural approach. But sure, we have bugs.

    Let me put the question back to you. Suppose a few OCFS2 bugs get fixed, do you think it would be able to handle what Dunnhumby is doing (see the original post) or say, even one of our Oil and Gas customers that have multiple clusters in the 250TB range slamming on seismic data? Remember, there are orders of magnitude more unstructured data than structured data so the term “general purpose” is not to be tossed about lightly.

    The odd paradox is why anyone would spend so much money on Oracle and then deploy it onto a cluster platform that is so unbelievably destabilizing (e.g., OCFS).

  3. 3 Mogens Nørgaard February 11, 2007 at 9:35 pm

    The joke about OCFS used to be that it should only be called OC, since it’s not a real File System :-))).

    Here’s one thing I really don’t like about the IT business: We are never really holding anyone accountable for, say, doing a party line for a certain product, then later (read: when a new version of a product is out) almost diss the old product.

    I remember a lot of Oracle folks telling everyone that OC(FS) was fantastic and that critics of it didn’t know what they were talking about.

    Now OCFS2 is out, and all these Oracle folks are talking about all the “well-known” issues/problems/bugs with OCFS1 – while maintaining that OC2 is the way and wave of the future.

    We should hold them responsible by, say, forcing them to run their home computers on OC2.

    Mogens

  4. 4 Noons February 11, 2007 at 11:33 pm

    you can’t hold them responsible, Moans. Impossible.

    The way it works is:

    – punter makes wild claims about untested, unreleased product.
    – the claims discredit any competition or different view of the world as “old”, or “out of touch with reality”, or a myriad other loads of crap.
    – once the product finally becomes available, punter quickly gets promoted or leaves to a better job, or joins Gartner.
    – try and pin it out to him now? “Sorry, punter doesn’t work with us anymore. But we have a much better grasp on reality now that we’ve restructured/renamed/re-architected that department”.

    Rinse and repeat.

    Yup: ain’t IT wonderful? And we wonder why it gets a bad name with CEOs?…


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