On 08/25/2009 01:52 PM, Andreas Dilger wrote:
On Aug 24, 2009 20:15 -0400, Theodore Ts'o wrote:
On Mon, Aug 24, 2009 at 05:43:36PM -0600, Andreas Dilger wrote:
Without transaction checksums waiting on all of the blocks together
is NOT safe. If the commit record is on disk, but the rest of the
transaction's blocks are not then during replay it may cause garbage
to be written from the journal into the filesystem metadata.
That's the one optimization we using journal checksums buys us.
Unfortunately it does not allow us to omit the barrier
operation.... and have real-world testing experience that without the
barrier, a power drop can cause significant filesystem corruption and
potential data loss.
Try using Chris Mason's torture-test workload with async-checksums
without this patch; you will get data corruption if you try dropping
power while his torture-test is running. I know you really don't like
the barrier, but I'm afraid it's not safe to run without it, even with
journal checksums.
In our performance testing of barriers (not with Chris' program), it
was FAR better to disable the disk cache and wait for IO completion
(i.e. barriers disabled) on just the journal blocks than to enable the
cache and cause a cache flush for each "barrier". The problem is that at
high IO rates there is much more data in the cache vs. the actual journal
blocks, and forcing the whole cache to be flushed each transaction commit
hurt our performance noticably.
Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.
This entirely depends on the nature of the storage device. If you are running on
any normal S-ATA drive, I have seen running with write cache disabled can cut
your large file write speed by half over running with barriers enabled.
Certainly less true when running with SAS or fibre channel devices.
Checking on ext4 on a newish Seagate 1TB disk, I see roughly parity (F12
rawhide, RC6 kernel):
[root@ricdesktop ~]# hdparm -W0 /dev/sdb
/dev/sdb:
setting drive write-caching to 0 (off)
write-caching = 0 (off)
[root@ricdesktop ~]# mkfs.ext4 /dev/sdb
mke2fs 1.41.8 (11-July-2009)
<snip>
[root@ricdesktop ~]# mount -o barrier=0 /dev/sdb /mnt/
[root@ricdesktop ~]# dd if=/dev/zero of=/mnt/bigfile bs=10M count=100
100+0 records in
100+0 records out
1048576000 bytes (1.0 GB) copied, 6.75127 s, 155 MB/s
[root@ricdesktop ~]# umount /mnt
[root@ricdesktop ~]# hdparm -W1 /dev/sdb
/dev/sdb:
setting drive write-caching to 1 (on)
write-caching = 1 (on)
[root@ricdesktop ~]# mount -o barrier=1 /dev/sdb /mnt/
[root@ricdesktop ~]# dd if=/dev/zero of=/mnt/bigfile bs=10M count=100
100+0 records in
100+0 records out
1048576000 bytes (1.0 GB) copied, 6.74861 s, 155 MB/s
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