[DRBD-user] DRBD very slow because of 4 KiB IO request size

Sebastian Riemer sebastian.riemer at profitbricks.com
Mon Sep 3 16:08:56 CEST 2012

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

On 09.07.2012 18:04, Sebastian Riemer wrote:
> Hi all,
> we at ProfitBricks do virtual data centers and we've got a customer who
> has built-up a DRBD 8.3.11 system inside his virtual data center.
> Storage is connected via InfiniBand and SCSI RDMA Protocol (SRP),
> presented as local storage in the VMs (VirtIO blk). Like with iSCSI and
> iSER such connections have trouble with 4k synchronous reads. The
> connection between VMs is done via VirtIO net but is also IB-based on
> the physical layer.
> The IO request size detection worked - the customer can do up to 128 KiB
> IO requests (not more because of the 128 KiB hashing functions) on
> regular data. But everything which is related to sync or resync is at
> only 4 KiB IO request size, verified by "blktrace". It is not a matter
> of the syncer rate limit. He has put it to 1000 MB/s. He can be lucky
> that he has 40 MB/s, because synchronous 4 KiB reads on 1 Gbit/s
> Ethernet with iSCSI are at only 10 MB/s.
> Why does DRBD syncs and resyncs in only slow 4 KiB IO requests?

Found the reason and the fix I guess:


It will be in 8.3.14.


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