Home > Articles > Operating Systems, Server > Solaris

This chapter is from the book

5.4 Causes of Read/Write File System Latency

There are several causes of latency in the file system read/write data path. The simplest is that of latency incurred by waiting for physical I/O at the backend of the file system. File systems, however, rarely simply pass logical requests straight through to the backend, so latency can be incurred in several other ways. For example, one logical I/O event can be fractured into two physical I/O events, resulting in the latency penalty of two disk operations. Figure 5.3 shows the layers that could contribute latency.


Figure 5.3 Layers for Observing File System I/O

Common sources of latency in the file system stack include:

  • Disk I/O wait (or network/filer latency for NFS)
  • Block or metadata cache misses
  • I/O breakup (logical I/Os being fractured into multiple physical I/Os)
  • Locking in the file system
  • Metadata updates

5.4.1 Disk I/O Wait

Disk I/O wait is the most commonly assumed type of latency problem. If the underlying storage is in the synchronous path of a file system operation, then it affects file-system-level latency. For each logical operation, there could be zero (a hit in a the block cache), one, or even multiple physical operations.

This iowait.d script uses the file name and device arguments in the I/O provider to show us the total latency accumulation for physical I/O operations and the breakdown for each file that initiated the I/O. See Chapter 4 for further information on the I/O provider and Section 10.6.1 for information on its arguments.

# ./iowait.d 639
Time breakdown (milliseconds):
 <on cpu>                                                       2478
 <I/O wait>                                                     6326

I/O wait breakdown (milliseconds):
 file1                                                           236
 file2                                                           241
 file4                                                           244
 file3                                                           264
 file5                                                           277
 file7                                                           330

5.4.2 Block or Metadata Cache Misses

Have you ever heard the saying "the best I/O is the one you avoid"? Basically, the file system tries to cache as much as possible in RAM, to avoid going to disk for repetitive accesses. As discussed in Section 5.6, there are multiple caches in the file system—the most obvious is the data block cache, and others include meta-data, inode, and file name caches.

5.4.3 I/O Breakup

I/O breakup occurs when logical I/Os are fractured into multiple physical I/Os. A common file-system-level issue arises when multiple physical I/Os result from a single logical I/O, thereby compounding latency.

Output from running the following DTrace script shows VOP level and physical I/Os for a file system. In this example, we show the output from a single read(). Note the many page-sized 8-Kbyte I/Os for the single 1-Mbyte POSIX-level read(). In this example, we can see that a single 1-MByte read is broken into several 4-Kbyte, 8-Kbyte, and 56-Kbyte physical I/Os. This is likely due to the file system maximum cluster size (maxcontig).

   # ./fsrw.d
Event           Device RW     Size Offset Path
sc-read              .  R  1048576      0 /var/sadm/install/contents
 fop_read            .  R  1048576      0 /var/sadm/install/contents
   disk_ra       cmdk0  R     4096     72 /var/sadm/install/contents
   disk_ra       cmdk0  R     8192     96 <none>
   disk_ra       cmdk0  R    57344     96 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    152 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    208 /var/sadm/install/contents
   disk_ra       cmdk0  R    49152    264 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    312 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    368 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    424 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    480 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    536 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    592 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    648 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    704 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    760 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    816 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    872 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    928 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344    984 /var/sadm/install/contents
   disk_ra       cmdk0  R    57344   1040 /var/sadm/install/contents

5.4.4 Locking in the File System

File systems use locks to serialize access within a file (we call these explicit locks) or within critical internal file system structures (implicit locks).

Explicit locks are often used to implement POSIX-level read/write ordering within a file. POSIX requires that writes must be committed to a file in the order in which they are written and that reads must be consistent with the data within the order of any writes. As a simple and cheap solution, many files systems simply implement a per-file reader-writer lock to provide this level of synchronization. Unfortunately, this solution has the unwanted side effect of serializing all accesses within a file, even if they are to non-overlapping regions. The reader-writer lock typically becomes a significant performance overhead when the writes are synchronous (issued with O_DSYNC or O_SYNC) since the writer-lock is held for the entire duration of the physical I/O (typically, in the order of 10 or more milliseconds), blocking all other reads and writes to the same file.

The POSIX lock is the most significant file system performance issue for databases because they typically use a few large files with hundreds of threads accessing them. If the POSIX lock is in effect, then I/O is serialized, effectively limiting the I/O throughput to that of a single disk. For example, if we assume a file system with 10 disks backing it and a database attempting to write, each I/O will lock a file for 10 ms; the maximum I/O rate is around 100 I/Os per second, even though there are 10 disks capable of 1000 I/Os per second (each disk is capable of 100 I/Os per second).

Most file systems using the standard file system page cache (see Section 14.7 in Solaris Internals) have this limitation. UFS when used with Direct I/O (see Section 5.6.2) relaxes the per-file reader-writer lock and can be used as a high-performance, uncached file system, suitable for applications such as databases that do their own caching.

5.4.5 Metadata Updates

File system metadata updates are a significant source of latency because many implementations synchronously update the on-disk structures to maintain integrity of the on-disk structures. There are logical metadata updates (file creates, deletes, etc.) and physical metadata updates (updating a block map, for example).

Many file systems perform several synchronous I/Os per metadata update, which limits metadata performance. Operations such as creating, renaming, and deleting files often exhibit higher latency than reads or writes as a result. Another area affected by metadata updates is file-extends, which can require a physical metadata update.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020