On 02/01/2012 06:41 PM, Chris Mason wrote:
On Wed, Feb 01, 2012 at 10:52:55AM -0600, James Bottomley wrote:
On Wed, 2012-02-01 at 11:45 -0500, Chris Mason wrote:
On Tue, Jan 31, 2012 at 11:28:26AM -0800, Gregory Farnum wrote:
On Tue, Jan 31, 2012 at 11:22 AM, Bernd Schubert
<bernd.schubert@xxxxxxxxxxxxxxxxxx> wrote:
I guess we should talk to developers of other parallel file systems and see
what they think about it. I think cephfs already uses data integrity
provided by btrfs, although I'm not entirely sure and need to check the
code. As I said before, Lustre does network checksums already and *might* be
interested.
Actually, right now Ceph doesn't check btrfs' data integrity
information, but since Ceph doesn't have any data-at-rest integrity
verification it relies on btrfs if you want that. Integrating
integrity verification throughout the system is on our long-term to-do
list.
We too will be said if using a kernel-level integrity system requires
using DIO, although we could probably work out a way to do
"translation" between our own integrity checksums and the
btrfs-generated ones if we have to (thanks to replication).
DIO isn't really required, but doing this without synchronous writes
will get painful in a hurry. There's nothing wrong with letting the
data sit in the page cache after the IO is done though.
I broadly agree with this, but even if you do sync writes and cache read
only copies, we still have the problem of how we do the read side
verification of DIX. In theory, when you read, you could either get the
cached copy or an actual read (which will supply protection
information), so for the cached copy we need to return cached protection
information implying that we need some way of actually caching it.
Good point, reading from the cached copy is a lower level of protection
because in theory bugs in your scsi drivers could corrupt the pages
later on.
But that only matters if the application is going to verify if data are
really on disk. For example (client server scenario)
1) client-A writes a page
2) client-B reads this page
client-B is simply not interested here where it gets the page from, as
long as it gets correct data. The network files system in between also
will just be happy existing in-cache crcs for network verification.
Only if the page is later on dropped from the cache and read again,
on-disk crcs matter. If those are bad, one of the layers is going to
complain or correct those data.
If the application wants to check data on disk it can either use DIO or
alternatively something like fadvsise(DONTNEED_LOCAL_AND_REMOTE)
(something I wanted to propose for some time already, at least I'm not
happy that posix_fadvise(POSIX_FADV_DONTNEED) is not passed to the file
system at all).
Cheers,
Bernd
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