[gpfsug-discuss] IO sizes

[Andrew Beattie]

Alex,

Metadata will be 4Kib 

Depending on the filesystem version you will also have subblocks to consider V4 filesystems have 1/32 subblocks, V5 filesystems have 1/1024 subblocks (assuming metadata and data block size is the same)

My first question would be is “ Are you sure that Linux OS is configured the same on all 4 NSD servers?.

My second question would be do you know what your average file size is if most of your files are smaller than your filesystem block size, then you are always going to be performing writes using groups of subblocks rather than a full block writes.

Regards, 

Andrew


> On 24 Feb 2022, at 04:39, Alex Chekholko <alex at calicolabs.com> wrote:
> 
> 
> This Message Is From an External Sender
> This message came from outside your organization.
> Hi,
> 
> Metadata I/Os will always be smaller than the usual data block size, right?
> Which version of GPFS?
> 
> Regards,
> Alex
> 
>> On Wed, Feb 23, 2022 at 10:26 AM Uwe Falke <uwe.falke at kit.edu> wrote:
>> 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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