On 10/30/2012 03:14 PM, NeilBrown wrote:

On Tue, 30 Oct 2012 13:14:24 -0700 "Darrick J. Wong"
[off-list ref] wrote:

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On Tue, Oct 30, 2012 at 08:19:41AM -0400, Martin K. Petersen wrote:

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"Neil" == NeilBrown  [off-list ref] writes:

<>

 So I see a need for 2 flags here.

Yes that was my thought too. We need two flags. The FS should communicate
it's capabilities as well.

 The first one is set by the device or transport to say "I would prefer
 stable writes if possible".
 The second is set by the filesystem, either because it has its own needs, or
 because it sees the first flag set on the device and chooses to honour it.
 The VFS/VM would act based on this second flag, and devices like md/RAID5
 would set the first flag, and assume writes are stable if the second flag is
 also set.

 This implies that setting that second flag must be handled synchronously by
 the filesystem, so that the device doesn't see the flag set until the
 filesystem has committed to honouring it.  That seems to make a mount (or
 remount) option the safest way to set it.

I think I do not like any mount option or any other tuneable. With the
block device stating it's needs and the FS confirming on it's capability
then I do not see how reverting that decision by admin can be any good.
Any overrides by an admin would then just be a bug.

 Comments?

NeilBrown

Thanks
Boaz