[DRBD-user] DRBD 8.2.6 - reason for full resync?!

Marcelo Azevedo marzevd at gmail.com
Thu Jun 26 13:22:08 CEST 2008


put :
  no-disk-flushes;
  no-md-flushes;
under ,  disk {  }
tell me if it makes a difference ...
On Tue, Jun 24, 2008 at 2:12 PM, Andrei Neagoe <anne at imc.nl> wrote:

> Thanks a lot for the clarification. That was exactly the case... from my
> understanding of the docs I thought it was just necessary to run drbdadm
> adjust all on each node, regardless of the node state (primary or
> secondary). Right now it's pretty clear how I must proceed with the testing.
> What still puzzles me is the fact that only one resource got the need to be
> fully resynchronized, because as I said, I'm running lvm2 over them (having
> drbd0 and drbd1 as physical volumes).
> Another thing is the speed, which atm it's let's say satisfactory, but I
> found a thread on linbit archive where a user with a very similar setup and
> testing scheme was getting ~37 MB/s over fiber link between 2 datacenters
> and if connected via crossover cable a transfer rate of almost 80 MB/s. You
> can view the thread here:
> http://archives.free.net.ph/message/20080523.225430.9ba8ceac.en.html
> Testing both network and writing to the external storage box directly
> reveals that these are not the limitations:
>
> *------------------------------------------------------------*
> *Client connecting to 10.0.0.20, TCP port 5001*
> *TCP window size: 0.02 MByte (default)*
> *------------------------------------------------------------*
> *[  3] local 10.0.0.10 port 39353 connected with 10.0.0.20 port 5001*
> *[ ID] Interval       Transfer     Bandwidth*
> *[  3]  0.0-10.0 sec  1125 MBytes    113 MBytes/sec*
> *[ ID] Interval       Transfer     Bandwidth*
> *[  3]  0.0-10.0 sec  1125 MBytes    112 MBytes/sec
> -----------------------------------------------------------
> [root at erebus testing]# dd if=/dev/zero of=test.dat bs=1G count=1
> oflag=dsync
> 1+0 records in
> 1+0 records out
> 1073741824 bytes (1.1 GB) copied, 10.321 seconds, 104 MB/s
>
> *
>
> Note that in the above test a different device is mounted in /testing (just
> another logical drive on the storage box). As an additional information, the
> storage box is an IBM DS 3200 connected to the machine using 2 SAS HBA's
> (just for redundancy, no load balancing).
>
> So at the moment I'm also pretty stuck with performance tuning as I don't
> know what else I could try.
>
> Thanks,
> Andrei Neagoe.
>
>
> Lars Ellenberg wrote:
>
> On Tue, Jun 24, 2008 at 12:27:30PM +0200, Andrei Neagoe wrote:
>
>
> Hi,
>
> I was trying today to play with drbd's settings and benchmark the results in
> order to obtain the best performance.
> Here is my test setup:
> 2 identical machines with sas storage boxes. Each machine has two 2TB device
> (in my case /dev/sdb and /dev/sdc) that I mirror over drbd and on top of them
> there's LVM set up. The nodes share a gbit link dedicated for drbd traffic.
> After the initial sync which took something around 20 hours to finish, I've
> created the LVM volume and formatted using ext3 FS. Then I started to play
> around with params like al-extents, unplug-watermark, maxbuffers, max-epoch by
> changing the  values and doing a drbdadm adjust all on each node (of course
> after copying the config file accordingly). In the begining it went pretty
> well, maximum value attained by dd test over drbd was 28.9 MB/s:
>
> [root at erebus testing]# dd if=/dev/zero of=test.dat bs=1G count=1 oflag=dsync
> 1+0 records in
> 1+0 records out
> 1073741824 bytes (1.1 GB) copied, 37.1114 seconds, 28.9 MB/s
>
> The configuration used is described in the end. After a couple more tests, I
> noticed a big impact on performance, getting around 19-20 MB/s so I checked /
> proc/drbd to see what's going on. Surprisingly, it was doing a full resync on
> one of the disks. Problem is, I don't understand why, as normally it should
> only resync discrepancies.
>
>
> if you change anything in the config file that changes "disk"
> parameters (like on-io-error, size, fencing, use-bmbv, ...),
> which causes drbdadm adjust to think it needs to detach/attach, and you
> do that while being primary, you get a full sync.
>
> this is unfortunate, and there should probably
> be a dialog to warn you about it.
>
> if you detach a Primary, then reattach, it will receive a full sync.
> you need to make it secondary first, if you want to avoid that.
> detaching, then reattaching a secondary will only receive an
> "incremental" resync, which typically is a few KB or nothing at all,
> depending on the timing.
>
> if this is not what happened for you, read the kernel log,
> typically drbd tells you why a resync was necessary.
>
>
> --
> : Lars Ellenberg                           http://www.linbit.com :
> : DRBD/HA support and consulting             sales at linbit.com :
> : LINBIT Information Technologies GmbH      Tel +43-1-8178292-0  :
> : Vivenotgasse 48, A-1120 Vienna/Europe     Fax +43-1-8178292-82 :
> __
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