Re: XFS peculiar behavior

[Eric Sandeen <sandeen@sandeen.net>]

Yannis Klonatos wrote:
> Hi all!
> 
>         I have come across the following peculiar behavior in XFS and i
> would appreciate any information anyone
> could provide.
>         In our lab we have a system that has twelve 500GByte hard disks
> (total capacity 6TByte), connected to an
> Areca (ARC-1680D-IX-12) SAS storage controller. The disks are configured
> as a RAID-0 device. Then I create
> a clean XFS filesystem on top of the raid volume, using the whole
> capacity. We use this test-setup to measure
> performance improvement for a TPC-H experiment. We copy the database
> over the clean XFS filesystem using the
> cp utility. The database used in our experiments is 56GBytes in size
> (data + indices).
>         The problem is that i have noticed that XFS may - not all times
> - split a table over a large disk distance. For
> example in one run i have noticed that a file of 13GByte is split over a
> 4,7TByte distance (I calculate this distance
> by subtracting the final block used for the file with the first one. The
> two disk blocks values are acquired using the
> FIBMAP ioctl).

xfs_bmap output would be a lot nicer.  Maybe you can paste that here to
show exactly what the layout is.

-Eric

>         Is there some reasoning behind this (peculiar) behavior? I would
> expect that since the underlying storage is so
> large, and the dataset is so small, XFS would try to minimize disk seeks
> and thus place the file sequentially in disk.
> Furthermore, I understand that there may be some blocks left unused by
> XFS between subsequent file blocks used
> in order to handle any write appends that may come afterward. But i
> wouldn't expect such a large splitting of a single
> file.
>         Any help?
> 
> Thanks in advance,
> Yannis Klonatos

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