[gpfsug-discuss] Confusing I/O Behavior

[Marc A Kaplan]

Good point about "tiny" files going into the inode and system pool.  Which 
reminds one:

   Generally a bad idea to store metadata in wide striping disk base RAID 
(Type 5 with spinning media) 
   Do use SSD or similar  for metadata.
   Consider smaller block size for metadata / system pool than regular 
file data.




From:   Bryan Banister <bbanister at jumptrading.com>
To:     gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Date:   04/11/2018 12:51 PM
Subject:        Re: [gpfsug-discuss] Confusing I/O Behavior
Sent by:        gpfsug-discuss-bounces at spectrumscale.org



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

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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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