[gpfsug-discuss] IO sizes

[Uwe Falke]

Dear all,

sorry for asking a question which seems not directly GPFS related:

In a setup with 4 NSD servers (old-style, with storage controllers in 
the back end), 12 clients and 10 Seagate storage systems, I do see in 
benchmark tests that  just one of the NSD servers does send smaller IO 
requests to the storage  than the other 3 (that is, both reads and 
writes are smaller).

The NSD servers form 2 pairs, each pair is connected to 5 seagate boxes 
( one server to the controllers A, the other one to controllers B of the 
Seagates, resp.).

All 4 NSD servers are set up similarly:

kernel: 3.10.0-1160.el7.x86_64 #1 SMP

HBA: Broadcom / LSI Fusion-MPT 12GSAS/PCIe Secure SAS38xx

driver : mpt3sas 31.100.01.00

max_sectors_kb=8192 (max_hw_sectors_kb=16383 , not 16384, as limited by 
mpt3sas) for all sd devices and all multipath (dm) devices built on top.

scheduler: deadline

multipath (actually we do have 3 paths to each volume, so there is some 
asymmetry, but that should not affect the IOs, shouldn't it?, and if it 
did we would see the same effect in both pairs of NSD servers, but we do 
not).

All 4 storage systems are also configured the same way (2 disk groups / 
pools / declustered arrays, one managed by  ctrl A, one by ctrl B,  and 
8 volumes out of each; makes altogether 2 x 8 x 10 = 160 NSDs).


GPFS BS is 8MiB , according to iohistory (mmdiag) we do see clean IO 
requests of 16384 disk blocks (i.e. 8192kiB) from GPFS.

The first question I have - but that is not my main one: I do see, both 
in iostat and on the storage systems, that the default IO requests are 
about 4MiB, not 8MiB as I'd expect from above settings (max_sectors_kb 
is really in terms of kiB, not sectors, cf. 
https://www.kernel.org/doc/Documentation/block/queue-sysfs.txt).

But what puzzles me even more: one of the server compiles IOs even 
smaller, varying between 3.2MiB and 3.6MiB mostly - both for reads and 
writes ... I just cannot see why.

I have to suspect that this will (in writing to the storage) cause 
incomplete stripe writes on our erasure-coded volumes (8+2p)(as long as 
the controller is not able to re-coalesce the data properly; and it 
seems it cannot do it completely at least)


If someone of you has seen that already and/or knows a potential 
explanation I'd be glad to learn about.


And if some of you wonder: yes, I (was) moved away from IBM and am now 
at KIT.

Many thanks in advance

Uwe


-- 
Karlsruhe Institute of Technology (KIT)
Steinbuch Centre for Computing (SCC)
Scientific Data Management (SDM)

Uwe Falke

Hermann-von-Helmholtz-Platz 1, Building 442, Room 187
D-76344 Eggenstein-Leopoldshafen

Tel: +49 721 608 28024
Email: uwe.falke at kit.edu
www.scc.kit.edu

Registered office:
Kaiserstraße 12, 76131 Karlsruhe, Germany

KIT – The Research University in the Helmholtz Association

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