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 31.100.01.00max_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: deadlinemultipath (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.fa...@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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