[DRBD-user] Slow Read performance

[Felix Frank]

Hi,

On 04/30/2011 11:12 PM, Maxim Ianoglo wrote:
> Hello,
> 
> Have two DELL PE R410, RAID 10 15K rpm Seagate SAS disks. Both servers
> are connected via 1Gb link.
> 
> Testing performance with bonnie++.
> On a RAW device  I get: ~240MB/s Read and ~390MB/s Write
> On DRBD device with Secondary DRBD host connected: ~80MB/s Read and
> ~340MB/s Write
> On DRBD without Secondary DRBD host connected: ~245MB/s Read and ~340Write

I'm not familiar with this tool. Does it mix reads and writes?

Looking at your write throughput being much larger than your read
throuhgput(!) I suspect you're writing to some sort of aggressive cache,
and your reads must wait until the writes are completed to the disk(?)
If that is indeed the case, this might explain why syncing to the
secondary slows down the read performance.
This seems especially likely since your write throughput is
a) not affected by live syncing and
b) WAY above the capacity of a 1GB link!

Long story short, I disbelieve your mode of testing is yielding results
of any operational value.

Also note that a volatile write cache may leave your secondary (and
primary) disks in a corrupted state on primary crash (and the secondary
even during connection loss). You may want to consider activating disk
flushes. Why does your config disable those?

> Why I get so bad Read performance on DRBD device with Secondary host
> connected ? Per documentation all reads should be made l

Yes they are local.

Also note:

>   syncer {
>     cpu-mask 4095;
>     rate 85M;
>     verify-alg "crc32c";
>     csums-alg "crc32c";
>   }

It is advisable to not use syncer rates that exceed 30% of your
bandwidth by much.

This rate is only used if an outdated peer is doing a full or partial
sync, it doesn't affect live synchronization (which is always done as
soon as each block is written).
If you use up most of your bandwidth for syncing, the write performance
on your DRBD will suffer.

HTH,
Felix