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

Bryan Banister bbanister at jumptrading.com
Mon Oct 6 17:30:02 BST 2014


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<mailto: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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