File system spanning multiple disks

David Brown davidb at davidb.org
Thu Aug 30 10:12:41 PDT 2012


On Fri, Jul 20, 2012 at 04:38:06PM -0700, Phil Karn wrote:
>On 7/20/12 11:03 AM, David Brown wrote:
>
>> Why would large file in XFS be better than using lvm?  lvm is fairly
>> easy to use, and works quite well.  There aren't barrier problems with
>> it any more, but I believe there still are with loopback files.
>
>I think of LVM as a special purpose file system that can handle multiple
>disks and allocate space to 'files' in one or more 4MiB extents, and its
>'files' appear as block special files rather than ordinary files. And at
>least until recently it introduced problems such as not passing write
>barriers through correctly.
>
>So why not just use a real file system unless LVM is somehow faster?

This just came up at the Linux Plumber's conference.  There were
basically two reasons given to not use loopback devices:

   - It breaks the kernel's caching of the data.  Basically, the logic
     that decies what should and shouldn't remain in the cache doesn't
     work very well through a loopback device.

   - Barriers don't get through, and they become much less reliable
     through bad powerdowns.

David

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