1 <?xml version='1.0' encoding='UTF-8'?>
2 <chapter xmlns="http://docbook.org/ns/docbook" xmlns:xl="http://www.w3.org/1999/xlink" version="5.0"
3 xml:lang="en-US" xml:id="understandinglustre">
4 <title xml:id="understandinglustre.title">Understanding Lustre Architecture</title>
5 <para>This chapter describes the Lustre architecture and features of Lustre. It includes the
6 following sections:</para>
10 <xref linkend="understandinglustre.whatislustre"/>
15 <xref linkend="understandinglustre.components"/>
20 <xref linkend="understandinglustre.storageio"/>
24 <section xml:id="understandinglustre.whatislustre">
26 <primary>Lustre</primary>
27 </indexterm>What a Lustre File System Is (and What It Isn't)</title>
28 <para>The Lustre architecture is a storage architecture for clusters. The central component of
29 the Lustre architecture is the Lustre file system, which is supported on the Linux operating
30 system and provides a POSIX<superscript>*</superscript> standard-compliant UNIX file system
32 <para>The Lustre storage architecture is used for many different kinds of clusters. It is best
33 known for powering many of the largest high-performance computing (HPC) clusters worldwide,
34 with tens of thousands of client systems, petabytes (PB) of storage and hundreds of gigabytes
35 per second (GB/sec) of I/O throughput. Many HPC sites use a Lustre file system as a site-wide
36 global file system, serving dozens of clusters.</para>
37 <para>The ability of a Lustre file system to scale capacity and performance for any need reduces
38 the need to deploy many separate file systems, such as one for each compute cluster. Storage
39 management is simplified by avoiding the need to copy data between compute clusters. In
40 addition to aggregating storage capacity of many servers, the I/O throughput is also
41 aggregated and scales with additional servers. Moreover, throughput and/or capacity can be
42 easily increased by adding servers dynamically.</para>
43 <para>While a Lustre file system can function in many work environments, it is not necessarily
44 the best choice for all applications. It is best suited for uses that exceed the capacity that
45 a single server can provide, though in some use cases, a Lustre file system can perform better
46 with a single server than other file systems due to its strong locking and data
48 <para>A Lustre file system is currently not particularly well suited for
49 "peer-to-peer" usage models where clients and servers are running on the same node,
50 each sharing a small amount of storage, due to the lack of Lustre-level data replication. In
51 such uses, if one client/server fails, then the data stored on that node will not be
52 accessible until the node is restarted.</para>
55 <primary>Lustre</primary>
56 <secondary>features</secondary>
57 </indexterm>Lustre Features</title>
58 <para>Lustre file systems run on a variety of vendor's kernels. For more details, see the
59 Lustre Test Matrix <xref xmlns:xlink="http://www.w3.org/1999/xlink"
60 linkend="dbdoclet.50438261_99193"/>.</para>
61 <para>A Lustre installation can be scaled up or down with respect to the number of client
62 nodes, disk storage and bandwidth. Scalability and performance are dependent on available
63 disk and network bandwidth and the processing power of the servers in the system. A Lustre
64 file system can be deployed in a wide variety of configurations that can be scaled well
65 beyond the size and performance observed in production systems to date.</para>
66 <para><xref linkend="understandinglustre.tab1"/> shows the practical range of scalability and
67 performance characteristics of a Lustre file system and some test results in production
70 <title xml:id="understandinglustre.tab1">Lustre Scalability and Performance</title>
72 <colspec colname="c1" colwidth="1*"/>
73 <colspec colname="c2" colwidth="2*"/>
74 <colspec colname="c3" colwidth="3*"/>
78 <para><emphasis role="bold">Feature</emphasis></para>
81 <para><emphasis role="bold">Current Practical Range</emphasis></para>
84 <para><emphasis role="bold">Tested in Production</emphasis></para>
92 <emphasis role="bold">Client Scalability</emphasis></para>
95 <para> 100-100000</para>
98 <para> 50000+ clients, many in the 10000 to 20000 range</para>
103 <para><emphasis role="bold">Client Performance</emphasis></para>
107 <emphasis>Single client: </emphasis></para>
108 <para>I/O 90% of network bandwidth</para>
109 <para><emphasis>Aggregate:</emphasis></para>
110 <para>2.5 TB/sec I/O</para>
114 <emphasis>Single client: </emphasis></para>
115 <para>2 GB/sec I/O, 1000 metadata ops/sec</para>
116 <para><emphasis>Aggregate:</emphasis></para>
117 <para>240 GB/sec I/O </para>
123 <emphasis role="bold">OSS Scalability</emphasis></para>
127 <emphasis>Single OSS:</emphasis></para>
128 <para>1-32 OSTs per OSS,</para>
129 <para>128TB per OST</para>
131 <emphasis>OSS count:</emphasis></para>
132 <para>500 OSSs, with up to 4000 OSTs</para>
136 <emphasis>Single OSS:</emphasis></para>
137 <para>8 OSTs per OSS,</para>
138 <para>16TB per OST</para>
140 <emphasis>OSS count:</emphasis></para>
141 <para>450 OSSs with 1000 4TB OSTs</para>
142 <para>192 OSSs with 1344 8TB OSTs</para>
148 <emphasis role="bold">OSS Performance</emphasis></para>
152 <emphasis>Single OSS:</emphasis></para>
153 <para> 5 GB/sec</para>
155 <emphasis>Aggregate:</emphasis></para>
156 <para> 2.5 TB/sec</para>
160 <emphasis>Single OSS:</emphasis></para>
161 <para> 2.0+ GB/sec</para>
163 <emphasis>Aggregate:</emphasis></para>
164 <para> 240 GB/sec</para>
170 <emphasis role="bold">MDS Scalability</emphasis></para>
174 <emphasis>Single MDS:</emphasis></para>
175 <para> 4 billion files</para>
177 <emphasis>MDS count:</emphasis></para>
178 <para> 1 primary + 1 backup</para>
179 <para condition="l24">Since Lustre release 2.4: up to 4096 MDSs and up to 4096
184 <emphasis>Single MDS:</emphasis></para>
185 <para> 750 million files</para>
187 <emphasis>MDS count:</emphasis></para>
188 <para> 1 primary + 1 backup</para>
194 <emphasis role="bold">MDS Performance</emphasis></para>
197 <para> 35000/s create operations,</para>
198 <para> 100000/s metadata stat operations</para>
201 <para> 15000/s create operations,</para>
202 <para> 35000/s metadata stat operations</para>
208 <emphasis role="bold">File system Scalability</emphasis></para>
212 <emphasis>Single File:</emphasis></para>
213 <para>2.5 PB max file size</para>
215 <emphasis>Aggregate:</emphasis></para>
216 <para>512 PB space, 4 billion files</para>
220 <emphasis>Single File:</emphasis></para>
221 <para>multi-TB max file size</para>
223 <emphasis>Aggregate:</emphasis></para>
224 <para>10 PB space, 750 million files</para>
230 <para>Other Lustre features are:</para>
233 <para><emphasis role="bold">Performance-enhanced ext4 file system:</emphasis> The Lustre
234 file system uses an improved version of the ext4 journaling file system to store data
235 and metadata. This version, called <emphasis role="italic"
236 ><literal>ldiskfs</literal></emphasis>, has been enhanced to improve performance and
237 provide additional functionality needed by the Lustre file system.</para>
240 <para><emphasis role="bold">POSIX standard compliance:</emphasis> The full POSIX test
241 suite passes in an identical manner to a local ext4 file system, with limited exceptions
242 on Lustre clients. In a cluster, most operations are atomic so that clients never see
243 stale data or metadata. The Lustre software supports mmap() file I/O.</para>
246 <para><emphasis role="bold">High-performance heterogeneous networking:</emphasis> The
247 Lustre software supports a variety of high performance, low latency networks and permits
248 Remote Direct Memory Access (RDMA) for InfiniBand<superscript>*</superscript> (utilizing
249 OpenFabrics Enterprise Distribution (OFED<superscript>*</superscript>) and other
250 advanced networks for fast and efficient network transport. Multiple RDMA networks can
251 be bridged using Lustre routing for maximum performance. The Lustre software also
252 includes integrated network diagnostics.</para>
255 <para><emphasis role="bold">High-availability:</emphasis> The Lustre file system supports
256 active/active failover using shared storage partitions for OSS targets (OSTs). Lustre
257 Release 2.3 and earlier releases offer active/passive failover using a shared storage
258 partition for the MDS target (MDT).</para>
259 <para condition="l24">With Lustre Release 2.4 or later servers and clients it is possible
260 to configure active/active failover of multiple MDTs. This allows application
261 transparent recovery. The Lustre file system can work with a variety of high
262 availability (HA) managers to allow automated failover and has no single point of
263 failure (NSPF). Multiple mount protection (MMP) provides integrated protection from
264 errors in highly-available systems that would otherwise cause file system
268 <para><emphasis role="bold">Security:</emphasis> By default TCP connections are only
269 allowed from privileged ports. UNIX group membership is verified on the MDS.</para>
272 <para><emphasis role="bold">Access control list (ACL), extended attributes:</emphasis> the
273 Lustre security model follows that of a UNIX file system, enhanced with POSIX ACLs.
274 Noteworthy additional features include root squash.</para>
277 <para><emphasis role="bold">Interoperability:</emphasis> The Lustre file system runs on a
278 variety of CPU architectures and mixed-endian clusters and is interoperable between
279 successive major Lustre software releases.</para>
282 <para><emphasis role="bold">Object-based architecture:</emphasis> Clients are isolated
283 from the on-disk file structure enabling upgrading of the storage architecture without
284 affecting the client.</para>
287 <para><emphasis role="bold">Byte-granular file and fine-grained metadata
288 locking:</emphasis> Many clients can read and modify the same file or directory
289 concurrently. The Lustre distributed lock manager (LDLM) ensures that files are coherent
290 between all clients and servers in the file system. The MDT LDLM manages locks on inode
291 permissions and pathnames. Each OST has its own LDLM for locks on file stripes stored
292 thereon, which scales the locking performance as the file system grows.</para>
295 <para><emphasis role="bold">Quotas:</emphasis> User and group quotas are available for a
296 Lustre file system.</para>
299 <para><emphasis role="bold">Capacity growth:</emphasis> The size of a Lustre file system
300 and aggregate cluster bandwidth can be increased without interruption by adding a new
301 OSS with OSTs to the cluster.</para>
304 <para><emphasis role="bold">Controlled striping:</emphasis> The layout of files across
305 OSTs can be configured on a per file, per directory, or per file system basis. This
306 allows file I/O to be tuned to specific application requirements within a single file
307 system. The Lustre file system uses RAID-0 striping and balances space usage across
311 <para><emphasis role="bold">Network data integrity protection:</emphasis> A checksum of
312 all data sent from the client to the OSS protects against corruption during data
316 <para><emphasis role="bold">MPI I/O:</emphasis> The Lustre architecture has a dedicated
317 MPI ADIO layer that optimizes parallel I/O to match the underlying file system
321 <para><emphasis role="bold">NFS and CIFS export:</emphasis> Lustre files can be
322 re-exported using NFS (via Linux knfsd) or CIFS (via Samba) enabling them to be shared
323 with non-Linux clients, such as Microsoft<superscript>*</superscript>
324 Windows<superscript>*</superscript> and Apple<superscript>*</superscript> Mac OS
325 X<superscript>*</superscript>.</para>
328 <para><emphasis role="bold">Disaster recovery tool:</emphasis> The Lustre file system
329 provides a distributed file system check (lfsck) that can restore consistency between
330 storage components in case of a major file system error. A Lustre file system can
331 operate even in the presence of file system inconsistencies, so lfsck is not required
332 before returning the file system to production.</para>
335 <para><emphasis role="bold">Performance monitoring:</emphasis> The Lustre file system
336 offers a variety of mechanisms to examine performance and tuning.</para>
339 <para><emphasis role="bold">Open source:</emphasis> The Lustre software is licensed under
340 the GPL 2.0 license for use with the Linux operating system.</para>
345 <section xml:id="understandinglustre.components">
347 <primary>Lustre</primary>
348 <secondary>components</secondary>
349 </indexterm>Lustre Components</title>
350 <para>An installation of the Lustre software includes a management server (MGS) and one or more
351 Lustre file systems interconnected with Lustre networking (LNET).</para>
352 <para>A basic configuration of Lustre components is shown in <xref
353 linkend="understandinglustre.fig.cluster"/>.</para>
355 <title xml:id="understandinglustre.fig.cluster">Lustre components in a basic cluster </title>
358 <imagedata scalefit="1" width="100%" fileref="./figures/Basic_Cluster.png"/>
361 <phrase> Lustre components in a basic cluster </phrase>
367 <primary>Lustre</primary>
368 <secondary>MGS</secondary>
369 </indexterm>Management Server (MGS)</title>
370 <para>The MGS stores configuration information for all the Lustre file systems in a cluster
371 and provides this information to other Lustre components. Each Lustre target contacts the
372 MGS to provide information, and Lustre clients contact the MGS to retrieve
374 <para>It is preferable that the MGS have its own storage space so that it can be managed
375 independently. However, the MGS can be co-located and share storage space with an MDS as
376 shown in <xref linkend="understandinglustre.fig.cluster"/>.</para>
379 <title>Lustre File System Components</title>
380 <para>Each Lustre file system consists of the following components:</para>
383 <para><emphasis role="bold">Metadata Server (MDS)</emphasis> - The MDS makes metadata
384 stored in one or more MDTs available to Lustre clients. Each MDS manages the names and
385 directories in the Lustre file system(s) and provides network request handling for one
386 or more local MDTs.</para>
389 <para><emphasis role="bold">Metadata Target (MDT</emphasis> ) - For Lustre Release 2.3 and
390 earlier, each file system has one MDT. The MDT stores metadata (such as filenames,
391 directories, permissions and file layout) on storage attached to an MDS. Each file
392 system has one MDT. An MDT on a shared storage target can be available to multiple MDSs,
393 although only one can access it at a time. If an active MDS fails, a standby MDS can
394 serve the MDT and make it available to clients. This is referred to as MDS
396 <para condition="l24">Since Lustre Release 2.4, multiple MDTs are supported. Each file
397 system has at least one MDT. An MDT on a shared storage target can be available via
398 multiple MDSs, although only one MDS can export the MDT to the clients at one time. Two
399 MDS machines share storage for two or more MDTs. After the failure of one MDS, the
400 remaining MDS begins serving the MDT(s) of the failed MDS.</para>
403 <para><emphasis role="bold">Object Storage Servers (OSS)</emphasis> : The OSS provides
404 file I/O service and network request handling for one or more local OSTs. Typically, an
405 OSS serves between two and eight OSTs, up to 16 TB each. A typical configuration is an
406 MDT on a dedicated node, two or more OSTs on each OSS node, and a client on each of a
407 large number of compute nodes.</para>
410 <para><emphasis role="bold">Object Storage Target (OST)</emphasis> : User file data is
411 stored in one or more objects, each object on a separate OST in a Lustre file system.
412 The number of objects per file is configurable by the user and can be tuned to optimize
413 performance for a given workload.</para>
416 <para><emphasis role="bold">Lustre clients</emphasis> : Lustre clients are computational,
417 visualization or desktop nodes that are running Lustre client software, allowing them to
418 mount the Lustre file system.</para>
421 <para>The Lustre client software provides an interface between the Linux virtual file system
422 and the Lustre servers. The client software includes a management client (MGC), a metadata
423 client (MDC), and multiple object storage clients (OSCs), one corresponding to each OST in
424 the file system.</para>
425 <para>A logical object volume (LOV) aggregates the OSCs to provide transparent access across
426 all the OSTs. Thus, a client with the Lustre file system mounted sees a single, coherent,
427 synchronized namespace. Several clients can write to different parts of the same file
428 simultaneously, while, at the same time, other clients can read from the file.</para>
429 <para><xref linkend="understandinglustre.tab.storagerequire"/> provides the requirements for
430 attached storage for each Lustre file system component and describes desirable
431 characteristics of the hardware used.</para>
433 <title xml:id="understandinglustre.tab.storagerequire"><indexterm>
434 <primary>Lustre</primary>
435 <secondary>requirements</secondary>
436 </indexterm>Storage and hardware requirements for Lustre components</title>
438 <colspec colname="c1" colwidth="1*"/>
439 <colspec colname="c2" colwidth="3*"/>
440 <colspec colname="c3" colwidth="3*"/>
444 <para><emphasis role="bold"/></para>
447 <para><emphasis role="bold">Required attached storage</emphasis></para>
450 <para><emphasis role="bold">Desirable hardware characteristics</emphasis></para>
458 <emphasis role="bold">MDSs</emphasis></para>
461 <para> 1-2% of file system capacity</para>
464 <para> Adequate CPU power, plenty of memory, fast disk storage.</para>
470 <emphasis role="bold">OSSs</emphasis></para>
473 <para> 1-16 TB per OST, 1-8 OSTs per OSS</para>
476 <para> Good bus bandwidth. Recommended that storage be balanced evenly across
483 <emphasis role="bold">Clients</emphasis></para>
489 <para> Low latency, high bandwidth network.</para>
495 <para>For additional hardware requirements and considerations, see <xref
496 linkend="settinguplustresystem"/>.</para>
500 <primary>Lustre</primary>
501 <secondary>LNET</secondary>
502 </indexterm>Lustre Networking (LNET)</title>
503 <para>Lustre Networking (LNET) is a custom networking API that provides the communication
504 infrastructure that handles metadata and file I/O data for the Lustre file system servers
505 and clients. For more information about LNET, see <xref
506 linkend="understandinglustrenetworking"/>.</para>
510 <primary>Lustre</primary>
511 <secondary>cluster</secondary>
512 </indexterm>Lustre Cluster</title>
513 <para>At scale, the Lustre cluster can include hundreds of OSSs and thousands of clients (see
514 <xref linkend="understandinglustre.fig.lustrescale"/>). More than one type of network can
515 be used in a Lustre cluster. Shared storage between OSSs enables failover capability. For
516 more details about OSS failover, see <xref linkend="understandingfailover"/>.</para>
518 <title xml:id="understandinglustre.fig.lustrescale"><indexterm>
519 <primary>Lustre</primary>
520 <secondary>at scale</secondary>
521 </indexterm>Lustre cluster at scale</title>
524 <imagedata scalefit="1" width="100%" fileref="./figures/Scaled_Cluster.png"/>
527 <phrase> Lustre cluster at scale </phrase>
533 <section xml:id="understandinglustre.storageio">
535 <primary>Lustre</primary>
536 <secondary>storage</secondary>
539 <primary>Lustre</primary>
540 <secondary>I/O</secondary>
541 </indexterm> Lustre Storage and I/O</title>
542 <para>In Lustre release 2.0, Lustre file identifiers (FIDs) were introduced to replace UNIX
543 inode numbers for identifying files or objects. A FID is a 128-bit identifier that contains a
544 unique 64-bit sequence number, a 32-bit object ID (OID), and a 32-bit version number. The
545 sequence number is unique across all Lustre targets in a file system (OSTs and MDTs). This
546 change enabled future support for multiple MDTs (introduced in Lustre release 2.3) and ZFS
547 (introduced in Lustre release 2.4).</para>
548 <para>Also introduced in 2.0 is a feature call <emphasis role="italic">FID-in-dirent</emphasis>
549 (also known as <emphasis role="italic">dirdata</emphasis>) in which the FID is stored as part
550 of the name of the file in the parent directory. This feature significantly improves
551 performance for <literal>ls</literal> command executions by reducing disk I/O. The
552 FID-in-dirent is generated at the time the file is created.</para>
554 <para>The FID-in-dirent feature is not compatible with the Lustre release 1.8 format.
555 Therefore, when an upgrade from Lustre release 1.8 to a Lustre release 2.x is performed, the
556 FID-in-dirent feature is not automatically enabled. For upgrades from Lustre release 1.8 to
557 Lustre releases 2.0 through 2.3, FID-in-dirent can be enabled manually but only takes effect
558 for new files. </para>
559 <para>For more information about upgrading from Lustre release 1.8 and enabling FID-in-dirent
560 for existing files, see <xref xmlns:xlink="http://www.w3.org/1999/xlink"
561 linkend="upgradinglustre"/>Chapter 16 “Upgrading a Lustre File System”.</para>
563 <para condition="l24">The LFSCK 1.5 file system administration tool released with Lustre release
564 2.4 provides functionality that enables FID-in-dirent for existing files. It includes the
565 following functionality:<itemizedlist>
567 <para>Generates IGIF mode FIDs for existing release 1.8 files.</para>
570 <para>Verifies the FID-in-dirent for each file to determine when it doesn’t exist or is
571 invalid and then regenerates the FID-in-dirent if needed.</para>
574 <para>Verifies the linkEA entry for each file to determine when it is missing or invalid
575 and then regenerates the linkEA if needed. The <emphasis role="italic">linkEA</emphasis>
576 consists of the file name plus its parent FID and is stored as an extended attribute in
577 the file itself. Thus, the linkEA can be used to parse out the full path name of a file
580 </itemizedlist></para>
581 <para>Information about where file data is located on the OST(s) is stored as an extended
582 attribute called layout EA in an MDT object identified by the FID for the file (see <xref
583 xmlns:xlink="http://www.w3.org/1999/xlink" linkend="Fig1.3_LayoutEAonMDT"/>). If the file is
584 a data file (not a directory or symbol link), the MDT object points to 1-to-N OST object(s) on
585 the OST(s) that contain the file data. If the MDT file points to one object, all the file data
586 is stored in that object. If the MDT file points to more than one object, the file data is
587 <emphasis role="italic">striped</emphasis> across the objects using RAID 0, and each object
588 is stored on a different OST. (For more information about how striping is implemented in a
589 Lustre file system, see <xref linkend="dbdoclet.50438250_89922"/>.</para>
590 <figure xml:id="Fig1.3_LayoutEAonMDT">
591 <title>Layout EA on MDT pointing to file data on OSTs</title>
594 <imagedata scalefit="1" width="80%" fileref="./figures/Metadata_File.png"/>
597 <phrase> Layout EA on MDT pointing to file data on OSTs </phrase>
601 <para>When a client wants to read from or write to a file, it first fetches the layout EA from
602 the MDT object for the file. The client then uses this information to perform I/O on the file,
603 directly interacting with the OSS nodes where the objects are stored.
604 <?oxy_custom_start type="oxy_content_highlight" color="255,255,0"?>This process is illustrated
605 in <xref xmlns:xlink="http://www.w3.org/1999/xlink" linkend="Fig1.4_ClientReqstgData"
606 /><?oxy_custom_end?>.</para>
607 <figure xml:id="Fig1.4_ClientReqstgData">
608 <title>Lustre client requesting file data</title>
611 <imagedata scalefit="1" width="75%" fileref="./figures/File_Write.png"/>
614 <phrase> Lustre client requesting file data </phrase>
618 <para>The available bandwidth of a Lustre file system is determined as follows:</para>
621 <para>The <emphasis>network bandwidth</emphasis> equals the aggregated bandwidth of the OSSs
622 to the targets.</para>
625 <para>The <emphasis>disk bandwidth</emphasis> equals the sum of the disk bandwidths of the
626 storage targets (OSTs) up to the limit of the network bandwidth.</para>
629 <para>The <emphasis>aggregate bandwidth</emphasis> equals the minimum of the disk bandwidth
630 and the network bandwidth.</para>
633 <para>The <emphasis>available file system space</emphasis> equals the sum of the available
634 space of all the OSTs.</para>
637 <section xml:id="dbdoclet.50438250_89922">
640 <primary>Lustre</primary>
641 <secondary>striping</secondary>
644 <primary>striping</primary>
645 <secondary>overview</secondary>
646 </indexterm> Lustre File System and Striping</title>
647 <para>One of the main factors leading to the high performance of Lustre file systems is the
648 ability to stripe data across multiple OSTs in a round-robin fashion. Users can optionally
649 configure for each file the number of stripes, stripe size, and OSTs that are used.</para>
650 <para>Striping can be used to improve performance when the aggregate bandwidth to a single
651 file exceeds the bandwidth of a single OST. The ability to stripe is also useful when a
652 single OST does not have enough free space to hold an entire file. For more information
653 about benefits and drawbacks of file striping, see <xref linkend="dbdoclet.50438209_48033"
655 <para>Striping allows segments or 'chunks' of data in a file to be stored on
656 different OSTs, as shown in <xref linkend="understandinglustre.fig.filestripe"/>. In the
657 Lustre file system, a RAID 0 pattern is used in which data is "striped" across a
658 certain number of objects. The number of objects in a single file is called the
659 <literal>stripe_count</literal>.</para>
660 <para>Each object contains a chunk of data from the file. When the chunk of data being written
661 to a particular object exceeds the <literal>stripe_size</literal>, the next chunk of data in
662 the file is stored on the next object.</para>
663 <para>Default values for <literal>stripe_count</literal> and <literal>stripe_size</literal>
664 are set for the file system. The default value for <literal>stripe_count</literal> is 1
665 stripe for file and the default value for <literal>stripe_size</literal> is 1MB. The user
666 may change these values on a per directory or per file basis. For more details, see <xref
667 linkend="dbdoclet.50438209_78664"/>.</para>
668 <para><xref linkend="understandinglustre.fig.filestripe"/>, the <literal>stripe_size</literal>
669 for File C is larger than the <literal>stripe_size</literal> for File A, allowing more data
670 to be stored in a single stripe for File C. The <literal>stripe_count</literal> for File A
671 is 3, resulting in data striped across three objects, while the
672 <literal>stripe_count</literal> for File B and File C is 1.</para>
673 <para>No space is reserved on the OST for unwritten data. File A in <xref
674 linkend="understandinglustre.fig.filestripe"/>.</para>
676 <title xml:id="understandinglustre.fig.filestripe">File striping on a Lustre file
680 <imagedata scalefit="1" width="100%" fileref="./figures/File_Striping.png"/>
683 <phrase>File striping pattern across three OSTs for three different data files. The file
684 is sparse and missing chunk 6. </phrase>
688 <para>The maximum file size is not limited by the size of a single target. In a Lustre file
689 system, files can be striped across multiple objects (up to 2000), and each object can be
690 up to 16 TB in size with ldiskfs. This leads to a maximum file size of 31.25 PB. (Note that
691 a Lustre file system can support files up to 2^64 bytes depending on the backing storage
692 used by OSTs.)</para>
694 <para>Versions of the Lustre software prior to Release 2.2 limited the maximum stripe count
695 for a single file to 160 OSTs.</para>
697 <para>Although a single file can only be striped over 2000 objects, Lustre file systems can
698 have thousands of OSTs. The I/O bandwidth to access a single file is the aggregated I/O
699 bandwidth to the objects in a file, which can be as much as a bandwidth of up to 2000
700 servers. On systems with more than 2000 OSTs, clients can do I/O using multiple files to
701 utilize the full file system bandwidth.</para>
702 <para>For more information about striping, see <xref linkend="managingstripingfreespace"