Hi,
Following the sf.net corruption report I've been checking our config
w.r.t data consistency. AFAIK the two main recommendations are:
1) don't mount FileStores with nobarrier
2) disable write-caching (hdparm -W 0 /dev/sdX) when using block dev
journals and your kernel is < 2.6.33
Obviously we don't do (1) because that would be crazy, but for (2) we
didn't disable yet write-caching, probably because we didn't notice
the doc.
But my lame excuse is that apparently _check_disk_write_cache in
FileJournal.cc doesn't print a warning when it should, because hdparm
-W doesn't always work on partitions rather than whole block devices.
See:
GOOD: ceph 0.94.2, kernel 3.10.0-229.7.2.el7.x86_64, hdparm v9.43:
10 journal _open_block_device: ignoring osd journal size. We'll use
the entire block device (size: 21474836480)
20 journal _check_disk_write_cache: disk write cache is on, but
your kernel is new enough to handle it correctly.
(fn:/var/lib/ceph/osd/ceph-96/journal)
1 journal _open /var/lib/ceph/osd/ceph-96/journal fd 20:
21474836480 bytes, block size 4096 bytes, directio = 1, aio = 1
BAD: ceph 0.94.2, kernel 2.6.32-431.29.2.el6.x86_64, hdparm v9.43:
10 journal _open_block_device: ignoring osd journal size. We'll use
the entire block device (size: 21474836480)
1 journal _open /var/lib/ceph/osd/ceph-56/journal fd 19:
21474836480 bytes, block size 4096 bytes, directio = 1, aio = 1
In other words, running hammer on EL6, _check_disk_write_cache exits
without printing anything, but actually it should log the scary
"WARNING: disk write cache is ON".
I guess it's because of this:
GOOD # uname -r && hdparm -W /dev/sda && hdparm -W /dev/sda1
3.10.0-229.7.2.el7.x86_64
/dev/sda1:
write-caching = 1 (on)
/dev/sda:
write-caching = 1 (on)
BAD # uname -r && hdparm -W /dev/sda && hdparm -W /dev/sda1
2.6.32-431.23.3.el6.x86_64
/dev/sda:
write-caching = 1 (on)
/dev/sda1:
HDIO_DRIVE_CMD(identify) failed: Inappropriate ioctl for device
(in both cases /dev/sda is an INTEL SSDSC2BA20).
So a few questions to end this:
1) What was the magic patch in 2.6.33 which made write-caching safe?
2) What's the recommended recourse here: hopefully Red Hat
backported the necessary to their 2.6.32 kernel, but if not should we
fix _check_disk_write_cache and make some publicity for people to
check their configs?
Best Regards,
Dan
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