Mailinglist Archive: opensuse (880 mails)

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[opensuse] Re: Slow file copying on openSUSE 12.3
Greg Freemyer wrote:

But with most setups barriers work and the journal can be relied on.
ext4 defaults to metadata journaling, but not data journalling.
That's the difference between write-back (data done after journaled
and after return success to user) vs. write-through (data is written through
to disk , synchronously with the journal (and /or before returning to user,
depending on usage context).
The read cache has other requirements. I constantly see:
[sda] Write cache: enabled, read cache: disabled, doesn't support DPO or FUA

on some of my disks on bootup.

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.
I am pretty sure this wasn't the reason -- but had more
to do with XFS's security requirement for Trusted Irix, which
required, in-its-design, for files with "unwritten extents" to
be marked as uninitialized -- so if they were mapped in the middle
of a file, they'd be zero'd before being handed to the user.

Conversely -- the opposite behavior, which wasn't as
famous was seen on most other file systems -- having random
garbage show up in a file -- that usually corresponded to a file
fragment of some previously deleted file. Only in a minority
of cases where such a file was a text file, would it be noted
as "file data from a foreign file". Even then, unless it belongs to
another user and contained something notable, someone
might have thought it was text from one of their files that
ended up as garbage in the middle of some other file...

That has been fixed
in XFS as far as I know for 5+ years now.
Having used XFS on linux on work and home PC's
since 2001, I'm pretty sure it was never the issue people
think it was but was more the case of them not noticing
equivalent cases of random-text in other file systems...
unless those systems did 'write-through' which gives
noticeably slow interactive performance.

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