[DRBD-user] Performance with DRBD + iSCSI

Ross S. W. Walker rwalker at medallion.com
Wed Feb 21 16:26:48 CET 2007

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


There has been a lot of back and forth on the lists about the
performance of iSCSI and DRBD together.

Let me post some information that I have learned through my use of both.

If you are performing disk io across a network then the performance of
your disk io is directly related to the performance of your network.

Discover the latency for 1 block sized packet of data to travel from one
host to another, say for a 4k packet.

24 packets transmitted, 24 received, 0% packet loss, time 23022ms
rtt min/avg/max/mdev = 0.164/0.226/0.290/0.038 ms, pipe 2

Ok, so for a 4096 sized packet it takes .226ms round trip. Looks like my
network needs some help here.

Now if I take 1000ms and divide it by .226 I get, 4425, that is the
maximum number of IOPS that my network setup can perform for this block
size, which at 4096 bytes equates to a maximum throughput of 18123894
bytes, or 17.2MB/s.

This is because each 4k io operation, the standard for file system
operations, needs to travel across the network. The lower you can get
those ping times, the higher your throughput will be up to what your
disk system can sustain at that block size.

Now this is true for DRBD by itself, or iSCSI by itself, but if you
combine the two then take whatever numbers you calculated above and
divide by 2 as each io will now have to travel across the network twice,
once from initiator to target, then again from target to replica.

Remember that for DRBD Prot C, synchronous io, that io operations will
not return (thus the next operation will not execute), until the write
operation has been committed to disk on both sides which means that
performance will be network bound.


Ross S. W. Walker
Information Systems Manager
Medallion Financial, Corp.
437 Madison Avenue
38th Floor
New York, NY 10022
Tel: (212) 328-2165
Fax: (212) 328-2125
WWW: http://www.medallion.com <http://www.medallion.com> 



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