[DRBD-user] write performance issues

Phil Stoneman pws at corefiling.com
Tue Jul 12 09:38:16 CEST 2011


On 11/07/11 22:07, Mark Dokter wrote:
> On 7/11/11 4:21 PM, Phil Stoneman wrote:
>> I think that LVM that comes with recent kernels support barriers, which
>> is why you're seeing this.
>
> How recent? I'm using the 2.6.32.41 xen-stable git tree from
> git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git

According to 
http://en.wikipedia.org/wiki/Logical_Volume_Manager_%28Linux%29#Caveats, 
write barriers in LVM were sort-of-supported from 2.6.31, and fully 
supported from 2.6.33.


>> Or, to put it another way: I have good and regularly tested backups; the
>> small risk of losing a bit of data on writes is more than offset by the
>> massive speed benefit that no-disk-barrier and no-disk-flushes gives me.
>>
>
> We've got a regular backup on tape, but that doesn't cover system
> partitions of the virtual machines. So the issue is the downtime, not
> the loss of data, if a partition gets corrupted :(

In our case, we want the advantages that drbd brings, but the department 
using that server (which is actually a KVM virtual machine) will notice 
if the performance is an order of magnitude worse than using local disks 
- which it sadly seems to be under some test conditions when using 
barriers :(

Phil



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