Re: Issue running buffered writes to a pNFS (NFS 4.1 backed by SAN) filesystem.

[Benjamin ESTRABAUD <be@mpstor.com>]

On 15/05/15 20:20, J. Bruce Fields wrote:
> On Fri, May 15, 2015 at 10:44:13AM -0700, Benjamin ESTRABAUD wrote:
>> I've been using pNFS for a while since recently, and I am very pleased
>> with its overall stability and performance.
>>
>> A pNFS MDS server was setup with SAN storage in the backend (a RAID0
>> built ontop of multiple LUNs). Clients were given access to the same
>> RAID0 using the same LUNs on the same SAN.
>>
>> However, I've been noticing a small issue with it that prevents me
>> from using pNFS to its full potential: If I run non-direct IOs (for
>> instance "dd" without the "oflag=direct" option), IOs run excessively
>> slowly (3-4MB/sec) and the dd process hangs until forcefully
>> terminated.
>
Sorry for the late reply, I was unavailable for the past few days. I had 
time to look at the problem further.

> And that's reproduceable every time?
>
It is, and here is what is happening more in details:

on the client, "/mnt/pnfs1" is the "pNFS" mount point. We use NFS v 4.1.

* Running dd with bs=512 and no "direct" set on the client:

dd if=/dev/zero of=/mnt/pnfs1/testfile bs=512 count=100000000

=> Here we get variable performance, dd's average is 100MB/sec, and we 
can see all the IOs going to the SAN block device. nfsstat confirms that 
no IOs are going through the NFS server (no "writes" are recorded, only 
"layoutcommit". Performance is maybe low but at this block size we don't 
really care.

* Running dd with bs=512 and "direct" setL

dd if=/dev/zero of=/mnt/pnfs1/testfile bs=512 count=100000000 oflag=direct

=> Here, funnily enough, all the IOs are sent over NFS. The "nfsstat" 
command shows writes increasing, the SAN block device activity on the 
client is idle. The performance is about 13MB/sec, but again expected 
with such a small IO size. The only unexpected is that small 512bytes 
IOs are not going through the iSCSI SAN.

* Running dd with bs=1M and no "direct" set on the client:

dd if=/dev/zero of=/mnt/pnfs1/testfile bs=1M count=100000000

=> Here the IOs "work" and go through the SAN (no "write" counter 
increasing in "nfsstat" and I can see disk statistics on the block 
device on the client increasing). However the speed at which the IOs go 
through is really slow (the actual speed recorded on the SAN device 
fluctuates a lot, from 3MB/sec to a lot more). Overall dd is not really 
happy and "Ctrl-C"ing it takes a long time, and in the last try actually 
caused a kernel panic (see http://imgur.com/YpXjvQ3 sorry about the 
picture format, did not have the dmesg output capturing and had access 
to the VGA only).
When "dd" finally comes around and terminates, the average speed is 
200MB/sec.
Again the SAN block device shows IOs being submitted and "nfsstat" shows 
no "writes" but a few "layoutcommits", showing that the writes are not 
going through the "regular" NFS server.


* Running dd with bs=1M and no "direct" set on the client:

dd if=/dev/zero of=/mnt/pnfs1/testfile bs=1M count=100000000 oflag=direct

=> Here the IOs work much faster (almost twice as fast as with "direct" 
set, or 350+MB/sec) and dd is much more responsive (can "Ctrl-C" it 
almost instantly). Again the SAN block device shows IOs being submitted 
and "nfsstat" shows no "writes" but a few "layoutcommits", showing that 
the writes are not going through the "regular" NFS server.

This shows that somehow running with "oflag=direct" causes unstability 
and lower performance, at least on this version.

Both clients are running Linux 4.1.0-rc2 on CentOS 7.0 and the server is 
running Linux 4.1.0-rc2 on CentOS 7.1.

> Can you get network captures and figure out (for example), whether the
> slow writes are going over iSCSI or NFS, and if they're returning errors
> in either case?
>
I'm going to do that now (try and locate errors). However "nfsstat" does 
indicate that slower writes are going through iSCSI.

>> The same behaviour can be observed laying out an IO file
>> with FIO for instance, or using some applications which do not use the
>> ODIRECT flag. When using direct IO I can observe lots of iSCSI
>> traffic, at extremely good performance (same performance as the SAN
>> gets on "raw" block devices).
>>
>> All the systems are running CentOS 7.0 with a custom kernel 4.1-rc2
>> (pNFS enabled) apart from the storage nodes which are running a custom
>> minimal Linux distro with Kernel 3.18.
>>
>> The SAN is all 40G Mellanox Ethernet, and we are not using the OFED
>> driver anywhere (Everything is only "standard" upstream Linux).
>
> What's the non-SAN network (that the NFS traffic goes over)?
>
The NFS traffic also goes through the same SAN actually, both the iSCSI 
LUNs and the NFS server are accessible over the same 40G/sec Ethernet 
fabric.

Regards,
Ben.

> --b.
>
>>
>> Would anybody have any ideas where this issue could be coming from?
>>
>> Regards, Ben - MPSTOR.-- To unsubscribe from this list: send the line
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