Archive for the 'High Performance Linux filesystem' Category

Apple OS/X with ZFS Shall Rule the World.

The problem with blogging is there is no way to clearly write something tongue-in-cheek. No facial expressions, no smirking, no hand waving—just plain old black and white. That aside, I can’t resist posting a follow-up to a post on StorageMojo about ZFS performance. I’m not taking a swipe at StorageMojo because it is one of my favorite blogs. However, every now and again third party perspective is a healthy thing. The topic at hand is this post about ZFS performance which is a digest of this post on digitalbadger.net. The original post starts with:

I have seen many benchmarks indicating that for general usage, ZFS should be at least as fast if not faster than UFS (directio not withstanding)[…]

Right off the bat I was scratching my head. I have experience with direct I/O dating back to 1991 and have some direct I/O related posts here. I was trying to figure out what the phrase “directio not withstanding” was supposed to mean. The only performance boost direct I/O can possibly yield is due to the elimination of the write-ordering locks generally imposed by POSIX and the double-buffering/memcopys overhead associated with the DMA into the page cache—and subsequent copy into the user address space buffer. So for a read-intensive benchmark, about the only thing one should expect is less processor overhead, not increased throughput.

Anyway, the post continues:

To give a little background : I have been experiencing really bad throughput on our 3510-based SAN. The hosts are X4100s, 12Gb RAM, 2x dual core 2.6Ghz opterons and Solaris 10 11/06. They are each connected to a 3510FC dual-controller array via a dual-port HBA and 2 Brocade SW200e switches, using MxPIO. All fabric is at 2Gb/s […]

OK, the 4100 is a 2-way Opteron 2000 server which, being a lot like an HP Proliant DL385, should have no problem consuming all the data the 2 x 2Gb FCP paths can deliver—roughly 400MB/s. If the post is about a UFS versus ZFS apples-apples comparison, both results should top out at the maximum theoretical throughput of 2x2Gb FCP. The post continues:

On average, I was seeing some pretty average to poor rates on the non-ZFS volumes depending on how they were configured, on average I was seeing 65MB/s write performance and around 450MB/s read on the SAN using a single drive LUN.

I’d be plenty happy with that 450MB/s given that configuration. The post insinuates this is direct I/O so getting 450MB/s out of 2x2Gb FCP should be the end of the contest. Further, getting 65MB/s written to a single drive LUN is quite snappy! What is left to benchmark? It seems UFS cranks the SAN at full bandwidth.

The post continues:

I then tried ZFS, and immediately started seeing a crazy rate of around 1GB/s read AND write, peaking at close to 2GB/s. Given that this was round 2 to 4 times the capacity of the fabric, it was clear something was going awry.

Hmmm…Opterons with HT 2.0 attaining peaks of 2GB/s write throughput via 2x2Gb FCP? There is nothing awry about this, it is simply not writing the data to the storage.

This is a memory test. ZFS is caching, UFS is not.

The post continues:

Many thanks to Greg Menke and Tim Bradshaw on comp.unix.solaris for their help in unravelling this mystery!

What mystery? I’d recommend testing something that runs longer than 1 second (reading 512MB from memory on any Opteron system should take about .6 seconds. I’d recommend a dataset that is about 2 fold larger than physical memory.

How Does ZFS and UFS Compare to Other Filesystems?
Let’s take a look at an equivalent test running on the HP Cluster Gateway internal filesystem (the former PolyServe PSFS). The following is a screen shot of a dd(1) test using real Oracle datafiles on a Proliant DL585. The first test executes a single thread of dd(1) reading the first 512MB of one of the files using 1M read requests. The throughput is 815MB/s. Next, I ran 2 concurrent dd(1) processes each chomping the first 512MB out of two different Oracle datafiles using 1MB read requests. In the concurrent case, the throughput was 1.46GB/s.

NOTE: You may have to right click->view the image.

zfsthing1.jpg

But wait, in order to prove the vast superiority of the HP Cluster Gateway filesystem over ZFS using similar hardware, I fired off 6 concurrent dd(1) processes each reading the first 512MB out of six different files. Here I measured 2.9GB/s!

zfsthing2.jpg

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.



							

Yes Direct I/O Means Concurrent Writes. Oracle Doesn’t Need Write-Ordering.

If Sir Isaac Newton was walking about today dropping apples to prove his theory of gravity, he’d feel about like I do making this blog entry. The topic? Concurrent writes on file system files with Direct I/O.

A couple of months back, I made a blog entry about BIGFILE tablespaces in ASM versus modern file systems.The controversy at hand at the time was about the dreadful OS locking overhead that must surely be associated with using large files in a file system. I spent a good deal of time tending to that blog entry pointing out that the world is no longer flat and such age-old concerns over OS locking overhead on modern file systems no longer relevant. Modern file systems support Direct I/O and one of the subtleties that seems to have been lost in the definition of Direct I/O is the elimination of the write-ordering locks that are required for regular file system access. The serialization is normally required so that if two processes should write to the same offset in the same file, one entire write must occur before the other—thus preventing fractured writes. With databases like Oracle, no two processes will write to the same offset in the same file at the same time. So why have the OS impose such locking? It doesn’t with modern file systems that support Direct I/O.

In regards to the blog entry called ASM is “not really an optional extra” With BIGFILE Tablespaces, a reader posted the following comment:

“node locks are only an issue when file metadata changes”
This is the first time I’ve heard this. I’ve had a quick scout around various sources, and I can’t find support for this statement.
All the notes on the subject that I can find show that inode/POSIX locks are also used for controlling the order of writes and the consistency of reads. Which makes sense to me….

Refer to:
http://www.ixora.com.au/notes/inode_locks.htm

Sec 5.4.4 of
http://www.phptr.com/articles/article.asp?p=606585&seqNum=4&rl=1

Sec 2.4.5 of
http://www.solarisinternals.com/si/reading/oracle_fsperf.pdf

Table 15.2 of
http://www.informit.com/articles/article.asp?p=605371&seqNum=6&rl=1

Am I misunderstanding something?

And my reply:

…in short, yes. When I contrast ASM to a file system, I only include direct I/O file systems. The number of file systems and file system options that have eliminated the write-ordering locks is a very long list starting, in my experience, with direct async I/O on Sequent UFS as far back as 1991 and continuing with VxFS with Quick I/O, VxFS with ODM, PolyServe PSFS (with the DBOptimized mount option), Solaris UFS post Sol8-U3 with the forcedirectio mount option and others I’m sure. Databases do their own serialization so the file system doing so is not needed.

The ixora and solarisinternals references are very old (2001/2002). As I said, Solaris 8U3 direct I/O completely eliminates write-ordering locks. Further, Steve Adams also points out that Solaris 8U3 and Quick I/O where the only ones they were aware of, but that doesn’t mean VxFS ODM (2001), Sequent UFS (starting in 1992) and ptx/EFS, and PolyServe PSFS (2002) weren’t all supporting completely unencumbered concurrent writes.

Ari, thanks for reading and thanks for bringing these old links to my attention. Steve is a fellow Oaktable Network Member…I’ll have to let him know about this out of date stuff.

There is way too much old (and incomplete) information out there.

A Quick Test Case to Prove the Point
The following screen shot shows a shell process on one of my Proliant DL585s with Linux RHEL 4 and the PolyServe Database Utility for Oracle. The session is using the PolyServe PSFS filesystem mounted with the DBOptimized mount option which supports Direct I/O. The test consists of a single dd(1) process overwriting the first 8GB of a file that is a little over 16GB. The first invocation of dd(1) writes 2097152 4KB blocks in 283 seconds for an I/O rate of 7,410 writes per second. The next test consisted of executing 2 concurrent dd(1) processes each writing a 4GB portion of the file. Bear in mind that the age old, decrepit write-ordering locks of yester-year serialized writes. Without bypassing those write locks, two concurrent write-intensive processes cannot scale their writes on a single file. The screen shot shows that the concurrent write test achieved 12,633 writes per second. Although 12,633 represents only 85% scale-up, remember, these are physical I/Os—I have a lot of lab gear, but I’d have to look around for a LUN that can do more than 12,633 IOps and I wanted to belt out this post. The point is that on a “normal” file system, the second go around of foo.sh with two dd(1) processes would take the same amount of time to complete as the single dd(1) run. Why? Because both tests have the same amount of write payload and if the second foo.sh suffered serialization the completion times would be the same:

conc_write2.JPG


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All content is © Kevin Closson and "Kevin Closson's Blog: Platforms, Databases, and Storage", 2006-2015. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Kevin Closson and Kevin Closson's Blog: Platforms, Databases, and Storage with appropriate and specific direction to the original content.

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