[DRBD-user] Writes are much too slow

Trent Lloyd lathiat at bur.st
Thu Sep 25 01:35:19 CEST 2008

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

Hi Artur,

On 25/09/2008, at 7:10 AM, eBoundHost: Artur wrote:

> Hi All,
> Putting together a system to compete with netapp and bluearc.  I say  
> 90% of it can be done with drbd and supermicro + LVM + ext3.
> Problem: slow write when server1 and server2 are connected.
> Here are my benchmarks:
> dd if=/dev/zero bs=4096 count=10000 oflag=dsync of=/data/file1
> DRBD Active and connected:
> 40960000 bytes (41 MB) copied, 24.8387 seconds, 1.6 MB/s
> DRBD Active and NOT connected:
> 40960000 bytes (41 MB) copied, 10.8259 seconds, 3.8 MB/s
> File system partition, NON DRBD, same disk:
> 40960000 bytes (41 MB) copied, 10.3686 seconds, 4.0 MB/s
> These are very consistent and repeatable.

I am in no way surprised by these results.  First note that you are  
writing out 4K blocks, then forcing a sync.  So you can see already  
this is a slow operation from the non-DRBD results.  As each time you  
write 4k you need to wait for it to sync before it can do the next 4k.

Problem with doing a force sync on DRBD is you introduce network  
latency into the 'sync' pipeline.. so it writes 4K.. sends a sync and  
you have to wait for that to go over the network, get synced, and come  
back, before it can move onto the next 4k block.

There isn't a whole lot you can do about this, but im not sure there  
is a 'fantastic' benchmark for real workload situations... anyway.

You could try some crazy things to reduce your latency.. remove  
switches try better drivers and stuff (does that really work!? - but  
ultimately I think you will need to stop doing 4k synced writes.. or  
get some low latency interconnect gear - but I suspect it's rather  

Something about this seem silly/wrong to you?

Trent Lloyd

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