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Top Forums UNIX for Beginners Questions & Answers Behaviour of echo commands used by Linux admins Post 303023904 by Peasant on Tuesday 25th of September 2018 10:49:30 AM
Old 09-25-2018
This is due sysfs being pseudo filesystem, as procfs is as well.

It's actually an interface from kernel space to user space which allows you to manipulate various options regarding your hardware or operating system values.
Options such as fibre rescan, ip4 forwarding etc are all controlled by user using those pseudo file systems.
A lot of those options have real backed configuration files (such as /etc/sysctl.conf) which will apply those on boot.

That file system is created after system is booted (or on request) and populated with your device files.
For instance, if you add additional fibre card, a sysfs will be populated with additional hierarchy for that card.

An actual echo command is predetermined, if you issue anything else error will be written.

Take the following example from my PC at home :
Code:
root# echo "- - -" > /sys/class/scsi_host/host1/scan
root# tail -f  /var/log/messages
kernel: [ 6598.240883] ata2: hard resetting link
kernel: [ 6598.555362] ata2: SATA link down (SStatus 0 SControl 300)
kernel: [ 6598.555382] ata2: EH complete
root# echo "gibberish" > /sys/class/scsi_host/host1/scan
-su: echo: write error: Invalid argument

Hope that helps
Regards
Peasant.
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HPSA(4) 						     Linux Programmer's Manual							   HPSA(4)

NAME
hpsa - HP Smart Array SCSI driver SYNOPSIS
modprobe hpsa [ hpsa_allow_any=1 ] DESCRIPTION
hpsa is a SCSI driver for HP Smart Array RAID controllers. Options hpsa_allow_any=1: This option allows the driver to attempt to operate on any HP Smart Array hardware RAID controller, even if it is not explicitly known to the driver. This allows newer hardware to work with older drivers. Typically this is used to allow installation of operating systems from media that predates the RAID controller, though it may also be used to enable hpsa to drive older controllers that would normally be handled by the cciss(4) driver. These older boards have not been tested and are not supported with hpsa, and cciss(4) should still be used for these. Supported hardware The hpsa driver supports the following Smart Array boards: Smart Array P700M Smart Array P212 Smart Array P410 Smart Array P410i Smart Array P411 Smart Array P812 Smart Array P712m Smart Array P711m StorageWorks P1210m Configuration details To configure HP Smart Array controllers, use the HP Array Configuration Utility (either hpacuxe(8) or hpacucli(8)) or the Offline ROM-based Configuration Utility (ORCA) run from the Smart Array's option ROM at boot time. FILES
Device nodes Logical drives are accessed via the SCSI disk driver (sd(4)), tape drives via the SCSI tape driver (st(4)), and the RAID controller via the SCSI generic driver (sg(4)), with device nodes named /dev/sd*, /dev/st*, and /dev/sg*, respectively. HPSA-specific host attribute files in /sys /sys/class/scsi_host/host*/rescan This is a write-only attribute. Writing to this attribute will cause the driver to scan for new, changed, or removed devices (e.g,. hot-plugged tape drives, or newly configured or deleted logical drives, etc.) and notify the SCSI midlayer of any changes detected. Normally a rescan is triggered automatically by HP's Array Configuration Utility (either the GUI or the command-line variety); thus, for logical drive changes, the user should not normally have to use this attribute. This attribute may be useful when hot plugging devices like tape drives, or entire storage boxes containing pre-configured logical drives. /sys/class/scsi_host/host*/firmware_revision This attribute contains the firmware version of the Smart Array. For example: # cd /sys/class/scsi_host/host4 # cat firmware_revision 7.14 HPSA-specific disk attribute files in /sys /sys/class/scsi_disk/c:b:t:l/device/unique_id This attribute contains a 32 hex-digit unique ID for each logical drive. For example: # cd /sys/class/scsi_disk/4:0:0:0/device # cat unique_id 600508B1001044395355323037570F77 /sys/class/scsi_disk/c:b:t:l/device/raid_level This attribute contains the RAID level of each logical drive. For example: # cd /sys/class/scsi_disk/4:0:0:0/device # cat raid_level RAID 0 /sys/class/scsi_disk/c:b:t:l/device/lunid This attribute contains the 16 hex-digit (8 byte) LUN ID by which a logical drive or physical device can be addressed. c:b:t:l are the controller, bus, target and lun of the device. For example: # cd /sys/class/scsi_disk/4:0:0:0/device # cat lunid 0x0000004000000000 Supported ioctl() operations For compatibility with applications written for the cciss(4) driver, many, but not all of the ioctls supported by the cciss(4) driver are also supported by the hpsa driver. The data structures used by these ioctls are described in the Linux kernel source file include/linux/cciss_ioctl.h. CCISS_DEREGDISK, CCISS_REGNEWDISK, CCISS_REGNEWD These three ioctls all do exactly the same thing, which is to cause the driver to rescan for new devices. This does exactly the same thing as writing to the hpsa-specific host "rescan" attribute. CCISS_GETPCIINFO Returns PCI domain, bus, device and function and "board ID" (PCI subsystem ID). CCISS_GETDRIVVER Returns driver version in three bytes encoded as: (major_version << 16) | (minor_version << 8) | (subminor_version) CCISS_PASSTHRU, CCISS_BIG_PASSTHRU Allows "BMIC" and "CISS" commands to be passed through to the Smart Array. These are used extensively by the HP Array Configuration Utility, SNMP storage agents, etc. See cciss_vol_status at <http://cciss.sf.net> for some examples. SEE ALSO
cciss(4), sd(4), st(4), cciss_vol_status(8), hpacucli(8), hpacuxe(8), <http://cciss.sf.net>, and Documentation/scsi/hpsa.txt and Documentation/ABI/testing/sysfs-bus-pci-devices-cciss in the Linux kernel source tree COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2012-08-05 HPSA(4)
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