[gpfsug-discuss] Confusing I/O Behavior

[Bryan Banister]

Just another thought here.  If the debug output files fit in an inode, then these would be handled as metadata updates to the inode, which is typically much smaller than the file system blocksize.  Looking at my storage that handles GPFS metadata shows avg KiB/IO at a horrendous 5-12 KiB!

HTH,
-B

-----Original Message-----
From: gpfsug-discuss-bounces at spectrumscale.org [mailto:gpfsug-discuss-bounces at spectrumscale.org] On Behalf Of Peter Serocka
Sent: Wednesday, April 11, 2018 6:07 AM
To: gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Subject: Re: [gpfsug-discuss] Confusing I/O Behavior

Note: External Email
-------------------------------------------------

Let’s keep in mind that line buffering is a concept
within the standard C library;
if every log line triggers one write(2) system call,
and it’s not direct io, then multiple write still get
coalesced into few larger disk writes (as with the dd example).

A logging application might choose to close(2)
a log file after each write(2) — that produces
a different scenario, where the file system might
guarantee that the data has been written to disk
when close(2) return a success.

(Local Linux file systems do not do this with default mounts,
but networked filesystems usually do.)

Aaron, can you trace your application to see
what is going on in terms of system calls?

— Peter


> On 2018 Apr 10 Tue, at 18:28, Marc A Kaplan <makaplan at us.ibm.com> wrote:
>
> Debug messages are typically unbuffered or "line buffered".   If that is truly causing a performance problem AND you still want to collect the messages -- you'll need to find a better way to channel and collect those messages.
>
>
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