[gpfsug-discuss] AFM limitations in a multi-cluster environment, slow prefetch operations

[Kalyan Gunda]

Hi Bryan,
 AFM supports GPFS multi-cluster..and we have customers already using this
successfully.  Are you using GPFS backend?
Can you explain your configuration in detail and if ls is hung it would
have generated some long waiters.  Maybe this should be pursued separately
via PMR.  You can ping me the details directly if needed along with opening
a PMR per IBM service process.

As for as prefetch is concerned, right now its limited to  one prefetch job
per fileset.  Each job in itself is multi-threaded and can use multi-nodes
to pull in data based on configuration.
"afmNumFlushThreads" tunable controls the number of threads used by AFM.
This parameter can be changed via mmchfileset cmd (mmchfileset pubs doesn't
show this param for some reason, I will have that updated.)

eg: mmchfileset fs1 prefetchIW -p afmnumflushthreads=5
Fileset prefetchIW changed.

List the change:
 mmlsfileset fs1 prefetchIW --afm -L
Filesets in file system 'fs1':

Attributes for fileset prefetchIW:
===================================
Status                                  Linked
Path                                    /gpfs/fs1/prefetchIW
Id                                      36
afm-associated                          Yes
Target
nfs://hs21n24/gpfs/fs1/singleTargetToUseForPrefetch
Mode                                    independent-writer
File Lookup Refresh Interval            30 (default)
File Open Refresh Interval              30 (default)
Dir Lookup Refresh Interval             60 (default)
Dir Open Refresh Interval               60 (default)
Async Delay                             15 (default)
Last pSnapId                            0
Display Home Snapshots                  no
Number of Gateway Flush Threads         5
Prefetch Threshold                      0 (default)
Eviction Enabled                        yes (default)

AFM parallel i/o can be setup such that multiple GW nodes can be used to
pull in data..more details are available here
http://www-01.ibm.com/support/knowledgecenter/SSFKCN_4.1.0/com.ibm.cluster.gpfs.v4r1.gpfs200.doc/bl1adv_afmparallelio.htm

and this link outlines tuning params for parallel i/o along with others:
http://www-01.ibm.com/support/knowledgecenter/SSFKCN_4.1.0/com.ibm.cluster.gpfs.v4r1.gpfs200.doc/bl1adv_afmtuning.htm%23afmtuning

Regards
Kalyan
GPFS Development
EGL D Block, Bangalore




From:	Bryan Banister <bbanister at jumptrading.com>
To:	gpfsug main discussion list <gpfsug-discuss at gpfsug.org>
Date:	10/06/2014 09:57 PM
Subject:	Re: [gpfsug-discuss] AFM limitations in a multi-cluster
            environment, slow prefetch operations
Sent by:	gpfsug-discuss-bounces at gpfsug.org



We are using 4.1.0.3 on the cluster with the AFM filesets,
-Bryan

From: gpfsug-discuss-bounces at gpfsug.org [
mailto:gpfsug-discuss-bounces at gpfsug.org] On Behalf Of Sven Oehme
Sent: Monday, October 06, 2014 11:28 AM
To: gpfsug main discussion list
Subject: Re: [gpfsug-discuss] AFM limitations in a multi-cluster
environment, slow prefetch operations

Hi Bryan,

in 4.1 AFM uses multiple threads for reading data, this was different in
3.5 . what version are you using ?

thx. Sven


On Mon, Oct 6, 2014 at 8:36 AM, Bryan Banister <bbanister at jumptrading.com>
wrote:
Just an FYI to the GPFS user community,

We have been testing out GPFS AFM file systems in our required process of
file data migration between two GPFS file systems.  The two GPFS file
systems are managed in two separate GPFS clusters.  We have a third GPFS
cluster for compute systems.  We created new independent AFM filesets in
the new GPFS file system that are linked to directories in the old file
system.  Unfortunately access to the AFM filesets from the compute cluster
completely hang.  Access to the other parts of the second file system is
fine.  This limitation/issue is not documented in the Advanced Admin Guide.

Further, we performed prefetch operations using a file mmafmctl command,
but the process appears to be single threaded and the operation was
extremely slow as a result.  According to the Advanced Admin Guide, it is
not possible to run multiple prefetch jobs on the same fileset:
GPFS can prefetch the data using the mmafmctl Device prefetch –j
FilesetName command (which specifies
a list of files to prefetch). Note the following about prefetching:
v It can be run in parallel on multiple filesets (although more than one
prefetching job cannot be run in
parallel on a single fileset).

We were able to quickly create the “--home-inode-file” from the old file
system using the mmapplypolicy command as the documentation describes.
However the AFM prefetch operation is so slow that we are better off
running parallel rsync operations between the file systems versus using the
GPFS AFM prefetch operation.

Cheers,
-Bryan




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