Delta IME

What is DDN IME?

Infinite Memory Engine (IME®) is a DDN solution for fast data tiering between the compute nodes and a file system in a high performance computing environment.

Conceptually the storage subsystem looks like the following

© DDN 2020

How to use IME

The preferred way to use the Delta IME is as a read-cache for frequently read data and as a write/read cache for small file io.

It is possible to use exiting utilities and applications with files residing or created on /ime. Performance will be equal to or better than using /scratch directly for i/o to files. 

To get additional performance from the IME software features without changing IO routines, use the posix2ime library (LD_PRELOAD'd), to intercept standard Posix IO calls with IME API calls.  We have included a module posix2ime that does this for you (read more about posix2ime below).

You can purge the contents of files from the cache , but not the presence of the file. Please see below. 

There are some important caveats when using the /ime file system for something other than a read-cache. See section 2.2 Data Consistency Model in the developer guide document .

Users must maintain close-to-open consistency when multiple clients access the same files. This requirement guarantees that any other client will see the latest changes made by one client as soon as the client opens the file. A client must synchronize all file data and metadata changes when it closes a file and unconditionally retrieve a file’s attributes when it opens a file, ignoring any information it may have cached about the file. IME implements an enhanced close-to-open consistency model, allowing IME to be lock free.

IME commands

Please see the man page for ime-ctl or the attached developer guide document for details.

Stage in and out single files

The ime-ctl command is used to stage and purge files from the caching /ime file system.

ime-ctl --prestage /ime/abcd/${USER}/file01

To sync the contents of a file created or changed that resides on /ime

ime-ctl --sync /ime/abcd/${USER}/file01

To purge the cached contents of a file on /ime

ime-ctl --purge /ime/abcd/${USER}/file01

Note that purging a file only clears the contents of the file from /ime. The /scratch and /ime file systems share the same name space which allows files and directories to be seen from either the caching front-end /ime or back-end /scratch.

Staging multiple files and directories

To recursively stage the contents of a directory and the files and directories below, in this case a directory called /scratch/abcd/${USER}/data_di, use the recursive 

ime-ctl --prestage --recursive --block /ime/abcd/${USER}/data_dir

The --block option makes sure the stage or sync is complete before returning.

Checking file stage/cache status

To check if a file has been staged to the IME cache in /ime or has its contents synced back to the back-end file system use the ime-ctl --frag-stat command.

In this example a file that was created as /scratch/abcd/${USER}/file01 that has not been staged to /ime. The file will be visible as /ime/abcd/${USER}/file01.

Not staged to /ime: all entries are showing "0" for the Dirty, Clean and Syncing entries. 

$ ime-ctl --frag-stat /ime/abcd/${USER}/file01
   File: `/ime/abcd/${USER}/file01'
         Number of bytes:
  Dirty: 0
  Clean: 0
Syncing: 0
Data on Slices:

After staging the file to /ime, the number of bytes in the "Clean" category show that the data on the cache is current.

$ ime-ctl --prestage /ime/abcd/${USER}/file01
$ ime-ctl --frag-stat /ime/abcd/${USER}/file01
   File: `/ime/abcd/${USER}/file01'
         Number of bytes:
  Dirty: 0
  Clean: 16777216
Syncing: 0
Data on Slices:  0

If the file /ime/abcd/${USER}/file01 was modified (appended, replaced, etc) one would see entries in the Dirty category:

$ ime-ctl --frag-stat /ime/abcd/${USER}/file01
   File: `/ime/abcd/${USER}/file01'
         Number of bytes:
  Dirty: 8388608
  Clean: 16777216
Syncing: 0
Data on Slices:  0

After using ime-ctl --sync  to flush the changes to the back-end file system, the dirty entries will be back to 0. 

$ ime-ctl --sync /ime/abcd/${USER}/file01
$ ime-ctl --frag-stat /ime/abcd/${USER}/file01
   File: `/ime/abcd/${USER}/file01'
         Number of bytes:
  Dirty: 0
  Clean: 25165824
Syncing: 0
Data on Slices:  0

IME posix2ime library

The posix2ime module is available and loading it will LD_PRELOAD the library for your shell or batch script and all subsequent commands.  The library is described at: DDNStorage/posix_2_ime: POSIX to IME Native API (github.com) . 

#!/bin/bash 
#SBATCH --mem=64g
#SBATCH --nodes=4
#SBATCH --ntasks-per-node=4
#SBATCH --exclusive
#SBATCH --cpus-per-task=16
#SBATCH --partition=cpu
#SBATCH --account=bbka-delta-cpu
#SBATCH --time=00:15:00
#SBATCH --job-name=posix2ime-ior-dedicated
 
BFS_DIR=/scratch/bbka/arnoldg/ime_example
IME_DIR=/ime/bbka/arnoldg/ime_example
SAMPLE_INPUT_FILE=myinputfile
 
# do many-files operations in /scratch before 
# using ime: cd $BFS_DIR; tar xvf inputbundle.tar ...

# bring the scratch directory into IME
ime-ctl --recursive --block --prestage $IME_DIR

# run the job/workflow in IME
# do serialized commands (avoiding many-files types of operations)
cd $IME_DIR
stat $SAMPLE_INPUT_FILE

# Use posix2ime for large block and/or parallel i/o 
module load posix2ime
time srun /u/arnoldg/ior/src/ior -F -b64m
# turn off posix2ime
unset LD_PRELOAD  # turns off posix2ime module

# synchronize IME back out to the Scratch directory ( $BFS_DIR )
ime-ctl --recursive --block --sync $IME_DIR

exit

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