Hi Uwe,
>> 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.
IMHO, If GPFS on this particular NSD server was restarted often during the
setup, then it is possible that the GPFS pagepool may not be contiguous. As a
result, GPFS 8MiB buffer in the pagepool might be a scatter-gather (SG) list
with many small entries (in the memory) resulting in smaller I/O when these
buffers are issued to the disks. The fix would be to reboot the server and
start GPFS so that pagepool is contiguous resulting in 8MiB buffer to be
comprised of 1 (or fewer) SG entries.
>>In the current situation (i.e. with IOs bit larger than 4MiB) setting
>>max_sectors_kB to 4096 might do the trick, but as I do not know the cause for
>>that behaviour it might well start to issue IOs >>smaller than 4MiB again at
>>some point, so that is not a nice solution.
It will be advised not to restart GPFS often in the NSD servers (in production)
to keep the pagepool contiguous. Ensure that there is enough free memory in NSD
server and not run any memory intensive jobs so that pagepool is not impacted
(e.g. swapped out).
Also, enable GPFS numaMemoryInterleave=yes and verify that pagepool is equally
distributed across the NUMA domains for good performance. GPFS
numaMemoryInterleave=yes requires that numactl packages are installed and then
GPFS restarted.
# mmfsadm dump config | egrep "numaMemory|pagepool "
! numaMemoryInterleave yes
! pagepool 282394099712
# pgrep mmfsd | xargs numastat -p
Per-node process memory usage (in MBs) for PID 2120821 (mmfsd)
Node 0 Node 1 Total
--------------- --------------- ---------------
Huge 0.00 0.00 0.00
Heap 1.26 3.26 4.52
Stack 0.01 0.01 0.02
Private 137710.43 137709.96 275420.39
---------------- --------------- --------------- ---------------
Total 137711.70 137713.23 275424.92
My two cents,
-Kums
Kumaran Rajaram
[cid:[email protected]]
From: [email protected]
<[email protected]> On Behalf Of Uwe Falke
Sent: Wednesday, February 23, 2022 8:04 PM
To: [email protected]
Subject: Re: [gpfsug-discuss] IO sizes
Hi,
the test bench is gpfsperf running on up to 12 clients with 1...64 threads
doing sequential reads and writes , file size per gpfsperf process is 12TB
(with 6TB I saw caching effects in particular for large thread numbers ...)
As I wrote initially: GPFS is issuing nothing but 8MiB IOs to the data disks,
as expected in that case.
Interesting thing though:
I have rebooted the suspicious node. Now, it does not issue smaller IOs than
the others, but -- unbelievable -- larger ones (up to about 4.7MiB). This is
still harmful as also that size is incompatible with full stripe writes on the
storage ( 8+2 disk groups, i.e. logically RAID6)
Currently, I draw this information from the storage boxes; I have not yet
checked iostat data for that benchmark test after the reboot (before, when IO
sizes were smaller, we saw that both in iostat and in the perf data retrieved
from the storage controllers).
And: we have a separate data pool , hence dataOnly NSDs, I am just talking
about these ...
As for "Are you sure that Linux OS is configured the same on all 4 NSD
servers?." - of course there are not two boxes identical in the world. I have
actually not installed those machines, and, yes, i also considered reinstalling
them (or at least the disturbing one).
However, I do not have reason to assume or expect a difference, the supplier
has just implemented these systems recently from scratch.
In the current situation (i.e. with IOs bit larger than 4MiB) setting
max_sectors_kB to 4096 might do the trick, but as I do not know the cause for
that behaviour it might well start to issue IOs smaller than 4MiB again at some
point, so that is not a nice solution.
Thanks
Uwe
On 23.02.22 22:20, Andrew Beattie wrote:
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
<[email protected]><mailto:[email protected]> wrote:
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 <[email protected]><mailto:[email protected]> wrote: Dear all, sorry
for asking a question which seems ZjQcmQRYFpfptBannerStart
This Message Is From an External Sender
This message came from outside your organization.
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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
<[email protected]<mailto:[email protected]>> 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<https://nam02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.kernel.org%2Fdoc%2FDocumentation%2Fblock%2Fqueue-sysfs.txt&data=04%7C01%7Ckrajaram%40geocomputing.net%7C52cc6360e6ea4be737ba08d9f7317d78%7C229a2792a5064f25b3bdbab585cec3ed%7C0%7C0%7C637812615096678246%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&sdata=II8k%2FHzrU7BC%2FVejg9AujgZGk1E0XTz8QCpH6IE6RGM%3D&reserved=0>).
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: [email protected]<mailto:[email protected]>
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--
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: [email protected]<mailto:[email protected]>
www.scc.kit.edu<https://nam02.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.scc.kit.edu%2F&data=04%7C01%7Ckrajaram%40geocomputing.net%7C52cc6360e6ea4be737ba08d9f7317d78%7C229a2792a5064f25b3bdbab585cec3ed%7C0%7C0%7C637812615096678246%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&sdata=mXwzkLB1EFB1Dh31rVRMwJZBY4CBbHcJduc9gK6M71A%3D&reserved=0>
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