Hi List,
I am using DRBD 8.3.7 with XenServer 5.5 U2. The drbd device is used by
3 different operating systems: Win XP, Win 2000 and Linux
paravirtualized 2.6.18.
I have an issue with concurrent local writes:
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120076912s +4096; pending: 120076912s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120076920s +4096; pending: 120076920s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120076928s +4096; pending: 120076928s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120076936s +4096; pending: 120076936s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120076944s +4096; pending: 120076944s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120076952s +4096; pending: 120076952s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120076960s +4096; pending: 120076960s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120076968s +4096; pending: 120076968s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120076976s +4096; pending: 120076976s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120076984s +4096; pending: 120076984s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120076992s +4096; pending: 120076992s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120077000s +4096; pending: 120077000s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120077008s +4096; pending: 120077008s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120077016s +4096; pending: 120077016s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 120077024s +4096; pending: 120077024s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121884912s +4096; pending: 121884912s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121884920s +4096; pending: 121884920s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121884928s +4096; pending: 121884928s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121884936s +4096; pending: 121884936s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121884944s +4096; pending: 121884944s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121884952s +4096; pending: 121884952s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121884960s +4096; pending: 121884960s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121884968s +4096; pending: 121884968s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121884976s +4096; pending: 121884976s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121884984s +4096; pending: 121884984s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121884992s +4096; pending: 121884992s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121885000s +4096; pending: 121885000s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121885008s +4096; pending: 121885008s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121885016s +4096; pending: 121885016s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121885024s +4096; pending: 121885024s +4096
block drbd1: tapdisk[22538] Concurrent local write detected! [DISCARD L]
new: 121885032s +4096; pending: 121885032s +4096
This event occurred in a period of 3 seconds and hasn't ever occurred
before or after.
This already has been discussed here:
http://lists.linbit.com/pipermail/drbd-user/2009-April/thread.html#11873
I understand the problem, however I have some questions:
1.) Lars Ellenberger said "DRBD currently _drops_ the later write.". Is
this behaviour identical to what any other block device would do? I
would guess that its safer to drop the first write attempt (or, if the
areas just overlap, create a single "combined" write attempt.)
2.) Why is this actually a problem when write barriers are used?
3.) Can drbd configured in such a way that the last write attempt is not
lost? I understand the possibility that an incorrect replication is
created but I prefer this over local data corruption and let a nightly
online verify take care of the differences.
4.) Is there any known switch in the configuration of windows, which
might prevent this issue (i.e. disabling the write cache?)?
5.) Before we used a software raid1 setup and noticed bitwise difference
of the disks when we executed a mdarray check. Maybe linux's software
raid suffers from the same problems as drbd. It has already been
discussed on various mailing lists:
http://www.mail-archive.com/[email protected]/msg27126.html
Greetings,
Alexander Thieme
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