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Re: [linux-lvm] LVM on SATA/PATA disks



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David Brown wrote:
> A simple lvm volume, mounted with XFS gives the following kernel
> message:
> 
>  Filesystem "dm-11": Disabling barriers, not supported by the underlying
> device
> XFS mounting filesystem dm-11

So it does. I don't use XFS on my boxes & hadn't noticed this before.

Turns out that xfs is checking barrier capabilities using queue->ordered
in the device struct. If it has QUEUE_ORDERED_NONE, barriers are
disabled (fs/xfs/linux-2.6/xfs_super.c).

I'll look into why this is the case as given this check there shouldn't
be any need for snap & mpath to check for barrier bios themselves.

The patches that introduced those checks were submitted after problems
were seen with barriers on these target types, so something was clearly
submitting them at some stage.

Kind regards,
Bryn.

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