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====== All About Lustre ======

The filesystems with which most users are familiar store data in discrete chunks called //blocks// on a single hard disk.  Blocks have a fixed size (for example, 4 KiB) so a 1 MiB file will be split into 256 blocks (256 blocks x 4 KiB/block = 1024 KiB).  Performance of this kind of filesystem depends on the speed at which blocks can be read from and written to the hard disk — (input/output, or i/o).  Different kinds of disk yield differing i/o performance:  a solid-state disk (SSD) will move blocks back and forth faster than a basic SATA hard disk used in a desktop computer.

Buying a better (and more expensive) disk is one way to improve i/o performance, but once the fastest, most expensive disk has been purchased this path leaves no room for further improvement.  The demands of an HPC cluster with several hundred (maybe even thousands) of compute nodes quickly outpaces the speed at which a single disk can shuttle bytes back and forth.  Parallelism saves the day:  store the filesystem blocks on more than one disk and the i/o performance of each will sum (to a degree).  For example, consider a computer that can move data to its hard disks in //1 cycle// with a hard disk that requires //4 cycles// to write a block.  Storing four blocks to just one hard disk would require 20 cycles: 1 cycle to move the block to the disk and 4 cycles to write it, with each block waiting on the completion of the previous:

{{ serial-vs-parallel.png?300 |Writing 4 blocks to (a) one disks and (b) four disks in parallel.}}

With four disks being used in parallel (example (b) above), the block writing overlaps and takes just 8 cycles to complete.

Parallel use of multiple disks is the key behind many higher-performance disk technologies.  RAID (Redundant Array of Independent Disks) level 6 uses three or more disks to improve i/o performance while retaining //parity// copies of data((The two parity copies in RAID-6 imply that given //N// 2 TB disks, only //N-2// actually store data.  E.g. a three disk RAID-6 volume has a capacity of 2 TB.)).  Should one or two of the constituent disks fail, the missing data can be reconstructed using the parity copies.  It is RAID-6 that forms the basic building block of the Lustre filesystem on the Mills cluster.

===== A Storage Node =====

The Mills cluster contains five //storage appliances// that each contain many hard disks.  For example, ''storage1'' contains 36 SATA hard disks (2 TB each) arranged as six 8 TB RAID-6 units:

{{ osts.png |The Mills storage1 appliance.}}

Each of the six OST (Object Storage Target) units can survive the concurrent failure of one or two hard disks at the expense of storage space:  the raw capacity of ''storage1'' is 72 TB, but the data resilience afforded by RAID-6 costs a full third of that capacity (leaving 48 TB).

The storage appliances are limited in their capabilities:  they only function to move blocks of data to and from the disks they contain.  In an HPC cluster the storage is shared between many compute nodes.  Nodes funnel their i/o requests to the shared storage system by way of the cluster's private network.  A dedicated machine called an OSS (Object Storage Server) acts as the middleman between the cluster nodes and the OSTs:

{{ oss-osts.png |Cluster nodes send i/o requests to an OSS, which services a set of OSTs.}}

In Mills, each OSS is primarily responsible for one storage appliance's OSTs.  As illustrated above, ''OST0000'' through ''OST0005'' are serviced primarily by ''OSS1''.  If ''OSS1'' were to fail compute nodes would no longer be able to interact with those OSTs.  This situation is tempered by having each OSS act as a //failover// OSS for a secondary set of OSTs.  If ''OSS1'' fails, then ''OSS2'' will take control of ''OST0000'' through ''OST0005'' in addition to its own ''OST0006'' through ''OST000B''.  When ''OSS1'' is repaired, it can retake control of its OSTs from its partner.

<note important>The action of an OSS's taking over for its failover partner is not immediate.  Usually anywhere from 5 to 10 minutes will pass before the partner OSS has fully assumed control over the OSTs.</note>

<note warning>Anytime an OST is unavailable, i/o operations destined for it will "hang" while they wait to be completed.  Users will perceive this hang in everything from file listings to reading from or writing to files residing on that OST.  You can check OST availability using the ''lfs check servers'' command.</note>

===== The Lustre Filesystem =====

As illustrated thus far, each OST increases i/o performance by simultaneously moving data blocks to the six hard disks of a RAID-6 unit.  Each OSS services six OSTs, accepting and interleaving six unique i/o workloads to further increase the speed with which data moves to and from the OSTs.  Having multiple OSSs (and thus additional OSTs) adds yet another level of parallelism to the scheme:  each OSS processes six unique i/o workloads.  The agglomeration of multiple OSS nodes, each servicing one or more OST, is the basis of a Lustre filesystem((The other half of the filesystem is the MDS (Meta-Data Server) and MDT (Meta-Data Target) which is constructed similarly to an OSS/OST but solely holds directory structure and file attributes like name, ownership, and permissions.)).

The benefits of a Lustre filesystem should be readily apparent from the discussion above:

  * Parallelism is leveraged at multiple levels to increase i/o performance
  * RAID-6 storage at the base level provides resilience
  * File system capacity is not limited by hard disk size

The capacity of a Lustre filesystem is the sum of its constituent OSTs, so a Lustre filesystem's capacity can be grown by the addition of OSTs (and possibly OSSs to serve them).  For example, should the 172 TB Lustre filesystem on Mills begin to approach its capacity, additional capacity could be added with zero downtime by buying and installing another OSS pair.

<note important>Creating extremely large filesystems has one drawback:  traversing the filesystem takes so much time that it becomes impossible to create off-site backups for further data resilience.  For this reason Lustre filesystems are most often treated as volatile/scratch storage.</note>

===== Further Performance Boost: Striping =====

Normally on a Lustre filesystem each file resides in toto on a single OST.  In Lustre terminology, a file maps to a single object, and an object is a variable-size chunk of data which resides on an OST.  When a program works with a file, it must direct all of its i/o requests to a single OST (and thus a single OSS).

For large files or files that are internally organized as "records((A //record// consists of a fixed-size sequence of bytes; the //i//-th record exists at an easily calculated offset within the file.))" i/o performance can be further improved by //striping// the file across multiple OSTs.  Striping divides a file into a set of sequential, fixed-size chunks.  The stripes are distributed round-robin to //N// unique Lustre objects -- and thus on //N// unique OSTs.  For example, consider a 13 MiB file:

{{ lustre-striping.png?500 |Lustre striping.}}

Without striping, all 13 MiB of the file resides in a single object on ''OST0001'' (see (a) above).  All i/o with respect to this file is handled by ''OSS1''; appending 5 MiB to the file will grow the object to 18 MiB.

With a stripe count of three and size of 4 MiB, the Lustre filesystem pre-allocates three objects on unique OSTs on behalf of the file (see (b) above).  The file is split into sequential segments of 4 MiB -- a stripe -- and the stripes are written round-robin to the objects allocated to the file.  In this case, appending 5 MiB to the file will see stripe 3 extended to a full 4 MiB and a new stripe of 2 MiB added to the object on ''OST0007''.  For large files and record-style files, striping introduces another level of parallelism that can dramatically increase the performance of programs that access them.

<note tip>File striping is established when the file is created.  Use the ''lfs setstripe'' command to pre-allocate the objects for a striped file:  ''lfs setstripe -c 4 -s 8m my_new_file.nc'' would create the file ''my_new_file.nc'' containing zero bytes with a stripe size (-s) of 8 MiB and striped across four objects (-c).</note>

<note tip>Once a file has been created its striping cannot be changed.  However, creating a new file with ''lfs setstripe'' and copying the contents of the old file into it effectively changes the data's striping pattern.</note>

===== Lustre utilities =====

There are two custom utilities, ''lrm'' and ''ldu'', available for removing files and checking disk usage on Lustre.   They both make use of specially-written code that rate-limits calls to the ''lstat()'', ''unlink()'' and ''rmdir()'' C functions to minimize the stress on Lustre.  Both of these utilities should be used on a compute node only (using ''qlogin'').

<note tip>If you will be processing relatively large directories with ''lrm'' or ''ldu'' it's a good idea to first start a ''screen'' session and then login to a compute node using ''qlogin''.</note>

==== Delete or remove (lrm) ====

''lrm'' is a custom utility available for removing files on Lustre. It reuses all but the ''%%--%%force'' flag of the native ''rm'' utility and reproduces its runtime behavior as closely as possible.  An additional option is present to track the size of all the removed items and report that at the end of the process. ''lrm'' should be used on a compute node only.

<code>
[traine@mills ~]$ lrm
usage:

  lrm {options} <path> {<path> ..}

 options:

  -h/--help                This information
  -V/--version             Version information
  -q/--quiet               Minimal output, please
  -v/--verbose             Increase the level of output to stderr as the program

  --interactive{=WHEN}     Prompt the user for removal of items.  Values for WHEN
                           are never, once (-I), or always (-i).  If WHEN is not
                           specified, defaults to always
  -i                       Shortcut for --interactive=always
  -I                       Shortcut for --interactive=once; user is prompted one time
                           only if a directory is being removed recursively or if more
                           than three items are being removed
  -r/--recursive           Remove directories and their contents recursively

  -s/--summary             Display a summary of how much space was freed...
    -k/--kilobytes         ...in kilobytes
    -H/--human-readable    ...in a size-appropriate unit

  -S/--stat-limit #.#      Rate limit on calls to stat(); floating-point value in
                           units of calls / second
  -U/--unlink-limit #.#    Rate limit on calls to unlink() and rmdir(); floating-
                           point value in units of calls / second
  -R/--rate-report         Always show a final report of i/o rates

 $Id: lrm.c 470 2013-08-22 17:40:01Z frey $
</code>

The example below shows user ''traine'' in workgroup ''it_nss'' on compute node ''n012'' removing ''/lustre/work/it_nss/projects/namd'' directory and all files and subdirectories using the ''%%--%%recursive'' option. The additional option ''%%--%%summary'' is also used to display how much space was freed in bytes. Note ''traine'' was already in ''/lustre/work/it_nss/projects'' before using ''qlogin'' to login into the compute node ''n012''.

<code>
[(it_nss:traine)@mills projects]$ qlogin
Your job 369292 ("QLOGIN") has been submitted
waiting for interactive job to be scheduled ...
Your interactive job 369292 has been successfully scheduled.
Establishing /opt/shared/OpenGridScheduler/local/qlogin_ssh session to host n012 ...
Last login: Thu Aug 22 14:32:16 2013 from mills.mills.hpc.udel.edu

[traine@n012 projects]$ pwd
/lustre/work/it_nss/projects

[traine@n012 projects]$ lrm --summary --recursive --stat-limit 100 --unlink-limit 100 ./namd
lrm: removed 354826645 bytes
</code>

<note important>Please set both limits (''%%--%%stat-limit'' and ''%%--%%unlink-limit'') to 100 in order to remove files at a slow pace on Lustre.</note>

==== Disk usage (ldu) ====

''ldu'' is a custom utility available for summarizing disk usage on Lustre. It reproduces its runtime behavior as closely as possible with the native ''du'' based on the options available below.  ''ldu'' should be used on a compute node only.

<code>
[traine@mills ~]$ ldu 
usage:

  ldu {options} <path> {<path> ..}

 options:

  -h/--help                This information
  -V/--version             Version information
  -q/--quiet               Minimal output, please
  -v/--verbose             Increase the level of output to stderr as the program

  -k/--kilobytes           Display usage sums in kilobytes
  -H/--human-readable      Display usage sums in a size-appropriate unit

  -S/--stat-limit #.#      Rate limit on calls to stat(); floating-point value in
                           units of calls / second
  -R/--rate-report         Always show a final report of i/o rates

 $Id: ldu.c 470 2013-08-22 17:40:01Z frey $
</code>

The example below shows user ''traine'' in workgroup ''it_nss'' on compute node ''n012'' summarizing their disk usage on ''/lustre/work/it_nss/projects'' directory. Note ''traine'' was already in ''/lustre/work/it_nss/projects'' before using ''qlogin'' to login into compute node ''n012''.

<code>
[(it_nss:traine)@mills projects]$ qlogin
Your job 369292 ("QLOGIN") has been submitted
waiting for interactive job to be scheduled ...
Your interactive job 369292 has been successfully scheduled.
Establishing /opt/shared/OpenGridScheduler/local/qlogin_ssh session to host n012 ...
Last login: Thu Aug 22 14:32:16 2013 from mills.mills.hpc.udel.edu

[traine@n012 projects]$ pwd
/lustre/work/it_nss/projects

[traine@n012 projects]$ ldu --human-readable --stat-limit 100 ./
[2013-08-22 13:48:00-0400] leon_stat:  25765 calls over 103.396 seconds (249 calls/sec)
[2013-08-22 13:48:37-0400] leon_stat:  25793 calls over 141.063 seconds (183 calls/sec)
[2013-08-22 13:49:19-0400] leon_stat:  25838 calls over 183.257 seconds (141 calls/sec)
[2013-08-22 13:50:43-0400] leon_stat:  26778 calls over 266.790 seconds (100 calls/sec)
821.07 GiB    /lustre/work/it_nss/projects
</code>

<note important>Please set the stat limit (''%%--%%stat-limit'') to 100 in order to summarize the disk usage on Lustre at a slow pace.

You may have noticed that the first rate shown above is NOT 100 as requested.  It takes 1 second for the rate-limiting logic to gather initial ''lstat()'', ''rmdir()'' and ''unlink()'' profiles.  After that, instantaneously meeting the desired rate would have the utility calling lstat() in bursts with long periods of inactivity between those bursts.  This is not the desired behavior.  Instead, the utility uses much shorter periods of inactivity to //eventually// meet the requested rate.</note>

<note tip>For long-running ''lrm'' or ''ldu'' cases, the ''USR1'' signal will cause the utility to display the current i/o rate(s) as seen in the above example.  The ''USR1'' signal can be delivered to the utility by finding its process id (using ''ps'') and then issuing the ''kill -USR1 <pid>'' command on the compute node on which ''lrm'' or ''ldu'' is running:<code bash>
[traine@n012 ~]$ ps -u traine
  PID TTY          TIME CMD
 7059 ?        00:00:00 sshd
 7060 pts/12   00:00:00 bash
 8834 pts/12   00:00:00 ps
 6000 ?        00:00:00 sshd
 6001 pts/12   00:00:00 bash
 6008 pts/12   00:00:00 ldu

[traine@n012 ~]$ kill -USR1 6008
</code></note>