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

Tue Oct 7 15:44:48 BST 2014

Interesting that AFM is supposed to work in a multi-cluster environment.  We were using GPFS on the backend.  The new GPFS file system was AFM linked over GPFS protocol to the old GPFS file system using the standard multi-cluster mount.   The "gateway" nodes in the new cluster mounted the old file system.  All systems were connected over the same QDR IB fabric.  The client compute nodes in the third cluster mounted both the old and new file systems.  I looked for waiters on the client and NSD servers of the new file system when the problem occurred, but none existed.  I tried stracing the `ls` process, but it reported nothing and the strace itself become unkillable.  There were no error messages in any GPFS or system logs related to the `ls` fail.  NFS clients accessing cNFS servers in the new cluster also worked as expected.  The `ls` from the NFS client in an AFM fileset returned the expected directory listing.  Thus all symptoms indicated the configuration wasn't supported.  I may try to replicate the problem in a test environment at some point.

However AFM isn't really a great solution for file data migration between file systems for these reasons:
1) It requires the complicated AFM setup, which requires manual operations to sync data between the file systems (e.g. mmapplypolicy run on old file system to get file list THEN mmafmctl prefetch operation on the new AFM fileset to pull data).  No way to have it simply keep the two namespaces in sync.  And you must be careful with the "Local Update" configuration not to modify basically ANY file attributes in the new AFM fileset until a CLEAN cutover of your application is performed, otherwise AFM will remove the link of the file to data stored on the old file system.  This is concerning and it is not easy to detect that this event has occurred.

2) The "Progressive migration with no downtime" directions actually states that there is downtime required to move applications to the new cluster, THUS DOWNTIME!  And it really requires a SECOND downtime to finally disable AFM on the file set so that there is no longer a connection to the old file system, THUS TWO DOWNTIMES!

3) The prefetch operation can only run on a single node thus is not able to take any advantage of the large number of NSD servers supporting both file systems for the data migration.  Multiple threads from a single node just doesn't cut it due to single node bandwidth limits.  When I was running the prefetch it was only executing roughly 100 " Queue numExec" operations per second.  The prefetch operation for a directory with 12 Million files was going to take over 33 HOURS just to process the file list!

4) In comparison, parallel rsync operations will require only ONE downtime to run a final sync over MULTIPLE nodes in parallel at the time that applications are migrated between file systems and does not require the complicated AFM configuration.  Yes, there is of course efforts to breakup the namespace for each rsync operations.  This is really what AFM should be doing for us... chopping up the namespace intelligently and spawning prefetch operations across multiple nodes in a configurable way to ensure performance is met or limiting overall impact of the operation if desired.

AFM, however, is great for what it is intended to be, a cached data access mechanism across a WAN.

Thanks,
-Bryan

-----Original Message-----
From: gpfsug-discuss-bounces at gpfsug.org [mailto:gpfsug-discuss-bounces at gpfsug.org] On Behalf Of Kalyan Gunda
Sent: Tuesday, October 07, 2014 12:03 AM
To: gpfsug main discussion list
Subject: Re: [gpfsug-discuss] AFM limitations in a multi-cluster environment, slow prefetch operations

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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