Red Hat APPLICATION STACK 1.1 RELEASE Manuale Utente Scaricare il pdf (Pagina 12)

Chapter 1: Software Overview

Tohelp alleviate this problem,StephenTweedie ofRedHat developed a mechanism that

allows disk I/O directly to a buffer in the application address space (historically known

as raw (or unprocessed) I/O). This mechanism will lock the required pages of memory

to prevent them from being paged out or swapped during the I/O operation.

Applications requiredto performthis typeof disk I/O would openthe characterspecial

device /dev/raw and bind the disk device to a special raw device using an ioctl(2) system

call.

Thismechanism,however,iscumbersometouseandsuffersfromsomedeﬁciencies.The

primary deﬁciency with the mechanism comes from its continued use of the ﬁle-system

buffer-headerdata structuresandassociateddevicequeueingroutines. Whileuseofthe

buffer headers is a straightforward mechanism, it implies that I/O operations will still

need to be fragmented into 1024-bytes per operation, increasing the kernel overhead

signiﬁcantly.The bindingmechanism used tobind an existingblockdevice to anew raw

device is also somewhat cumbersome and counterintuitive to Unix system

administrators, who expect to ﬁnd a relationship in the device namespace between a

block device and its corresponding raw device.

To address these concerns, SGI has added additional capabilities to Stephen Tweedie’s

raw I/O patch that allow large I/O operations directly to the user address space and

bypasses the bulk of the kernel I/O queueing code for SCSI and FiberChannel devices.

You can download a dd command that is capable of using the raw device features from

the following FTP location:

ftp://oss.sgi.com/projects/rawio/download/dd.raw

Thisfeatureis off by default,but youcan turnit onby settingthe CONFIG_RAWkernel

conﬁguration parameter.

More information about raw I/O is available from the following URL:

http://oss.sgi.com/projects/rawio/faq.html

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Red Hat APPLICATION STACK 1.1 RELEASE Manuale Utente Pagina 12