[DRBD-user] DRBD on top of LVM - 50% performace drop

Joshua West jwest at brandeis.edu
Sun Sep 12 01:31:04 CEST 2010

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


Are you making use of LVM snapshots by any chance?  I've read reports of 
extremely awful performance as a result of LVM2 CoW code -- whether 
you're on the trunk volume or any snapshot volumes:


1GB took 20sec to write w/o snapshots, 16min with snapshots!

If you're using LVM snapshots on that host, then perhaps that explains 
the performance penalties you've noticed.

On 09/10/10 06:47, trekker.dk at abclinuxu.cz wrote:
> Hello,
> I'm trying to use DRBD as storage for virtualised guests. DRBD is 
> created on top of LVM partition, all LVM partitions reside on single 
> software RAID1 array (2 disks)
> In this setup the DRBD is supposed to operate in standalone mode most 
> of the time, network connections will only come into play when 
> migrating a guest to another host (That's why I can't use RAID - DRBD 
> - LVM - there can be more than 2 hosts and guests need to be able to 
> migrate anywhere.)
> So I did very simple benchmark, created an LVM partition and tried to 
> write into it and then read the data:
> # dd if=/dev/zero of=/dev/mapper/obrazy2-pokus \
>  bs=$((1024**2)) count=16384
> # dd if=/dev/mapper/obrazy2-pokus of=/dev/null \
>  bs=$((1024**2)) count=16384
> Both tests yielded about 80MB/s throughput.
> Then I created a DRBD on top of that LVM and retried the test:
> # /sbin/drbdmeta 248 v08 /dev/mapper/obrazy2-pokus internal create-md
> # drbdsetup 0 disk \
>  /dev/mapper/obrazy2-pokus /dev/mapper/obrazy2-pokus \
>  internal --set-defaults --create-device
> # drbdsetup 0 primary -o
> Reading performace was the same, but writes dropped to about 35MB/s, 
> with flushes disabled (drbdsetup ... -a -i -D -m) about 45MB/s.
> I'd understand that, if the device was connected over network, but in 
> standalone mode I was expecting DRBD to have roughly the same 
> performance as underlying storage.
> My question: is that drop in write throughput normal or could there be 
> some error in my setup, which is causing it?
> System setup is: Debian, kernel 2.6.34 from kernel.org, drbd-utils 
> 8.3.7. Also tested on kernel from kernel.org.
> Regards,
> J.B.
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> drbd-user at lists.linbit.com
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Joshua West
Senior Systems Engineer
Brandeis University

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