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Are Linux File Systems Right for You? Page 2

By Henry Newman (Send Email)
Posted May 12, 2008


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The goals for Linux file systems and the Linux kernel design seem to be trying to address a completely different set of problems than AIX or Solaris, and IBM and Sun are far more directly responsible than the Linux community if you have a problem. If you run AIX or Solaris and complain to IBM or Sun, they can't say we have no control.

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Linux File Systems

Remember that most Linux file systems were designed around replacing NTFS, not some of the high-performance file systems such as GPFS (IBM), StorNext (Quantum) or QFS (Sun). These file systems were designed for streaming I/O, which we now know is important for everyone and for some high-speed IOPS, and in some cases for database access.

The Linux file systems that are commonly used today (ext-3 today and likely soon ext-4 and xfs) have not had huge structural changes in a long time. Ext-4 improves on ext-3 and ext-2 for some improved allocation, but simple things like alignment of the superblock to the RAID stripe and the first metadata allocation are not considered.

Additionally, things like alignment of additional file system metadata regions to RAID stripe value are not considered, nor are simple things like indirect allocations, which are fixed values so with the small allocations supported (4 KB maximum), large numbers of allocations are required. Take a 200 TB file system, which will require 53.7 billion allocations to represent the 200 TB using the largest allocation size of 4 KB supported by ext-3. Using 8 MB, which is feasible on enterprise file systems, it becomes a manageable 26.2 million allocations. The bitmap or allocation map could even fit in memory for this number of allocations! The xfs file system has very similar characteristics to ext-3. Yes, allocations can be larger, up to 64 KB if the Linux page size is 64 KB, but the alignment issues for the superblock, metadata regions and other issues still exist.

Linux Has Its Place

None of this is to say I am anti-Linux; nor am I pro-AIX or pro-Solaris. I am not even anti-Windows, since I use a Windows laptop as my main computer. But I do believe that the default Linux file systems are not yet up to the task of replacing the high-performance, highly scalable SMP file systems. Computers are tools, and operating systems and file systems are also tools in the toolbox. No one should use a chainsaw in place of a jigsaw, and the same analogy can be used for operating systems, file systems and the hardware on which they run.

Many of the people I deal with daily use MS Word, MS Excel, MS PowerPoint and MS Visio. I could run some if not all of these applications on a Windows emulator from someone, but I routinely get incompatibilities with fonts, and I just decided long ago to live with Windows until someone can prove to me that it all works together with no problems. My point here is that every computer is a tool and has its use. Currently no single computer or file system can meet all application requirements. This should not come as a surprise. Linux has a place, but as far as I can tell, that place does not support single instances of large file systems and scaling well from large to small file systems with high-performance requirements. And I don't see this changing any time soon.

Henry Newman is a regular contributor to Enterprise Storage Forum, where this story originally appeared. Newman is an industry consultant with 27 years experience in high-performance computing and storage.

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