Mailinglist Archive: opensuse (880 mails)

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Re: [opensuse] Slow file copying on openSUSE 12.3
On Thu, Aug 15, 2013 at 10:34 AM, James Knott <james.knott@xxxxxxxxxx> wrote:
Per Jessen wrote:
No, power loss is the critical event - as far as Linux is concerned, the
data has been written when the disk confirms, but until the data has
actually made it to a platter, a power loss would be bad.

Isn't the purpose of journaling to prevent this sort of problem? If
it's not in the journal, it hasn't been written.

But you have to ensure the journal write get to disk, not stuck in a
cache. I'm pretty sure XFS and ext4 both use "barriers" to make that
happen as appropriate. But those barriers have to be supported all
the way to the drive. mdraid use to drop them on the floor. Some
sata drives drop them on the floor.

But with most setups barriers work and the journal can be relied on.
ext4 defaults to metadata journaling, but not data journalling. That
means your filesystem structure is protected by barriers, but the
content of your files is not. XFS for years was renowned for
protecting files full of nulls for this reason. That has been fixed
in XFS as far as I know for 5+ years now.

Greg
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