10-17-2012
Well, I am sure most SAN can be configured for mirroring, which is raid-2 or something like that. To achieve the most bang for your buck, you want the mirrors to be as far apart as possible, so something between the application host and short of the storage box needs to know where there are 2 so it can sidestep the dead side as well as splitting the query load and duplicating the churn. The farther upstream it is done, the greater the reliability, but too close can load the app server and communications, unless fiber has multicast write and anycast read. There may be several layers vying to mirror your storage, so pick wisely.
Mirroring got overshadowed a bit by raid, but has always had a query bandwidth advantage, with two devices handling query. Within the devices, there can be as much striping as in raid, so that is no different. When writing, there is no parity calculation and additional write time, just two immediate simultaneous writes. With raid in sequential striping mode, you write data to 1,2,3,4 and parity to 5, then data to 5,1,2,3 and parity to 4, and so on, so while reading is 5x spindle speed, writing is 4x. A mirrored pair trades space for bandwidth. Disk is cheap, and bandwidth is golden. Finally, it seems some raid systems seem to only get defects detected by staff when 2 adjacent devices fail, so often raid5 is either also mirrored or great downtime, data loss and partial restore pain is experienced.
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LEARN ABOUT REDHAT
mkinitrd
MKINITRD(8) System Manager's Manual MKINITRD(8)
NAME
mkinitrd - creates initial ramdisk images for preloading modules
SYNOPSIS
mkinitrd [--version] [-v] [-f]
[--preload=module] [--omit-scsi-modules]
[--omit-raid-modules] [--omit-lvm-modules]
[--with=module] [--image-version]
[--fstab=fstab] [--nocompress]
[--builtin=module] [--nopivot]
image kernel-version
DESCRIPTION
mkinitrd creates filesystem images which are suitable for use as Linux initial ramdisk (initrd) images. Such images are often used for
preloading the block device modules (such as IDE, SCSI or RAID) which are needed to access the root filesystem. mkinitrd automatically
loads filesystem modules (such as ext3 and jbd), IDE modules, all scsi_hostadapter entries in /etc/modules.conf, and raid modules if the
system's root partition is on raid, which makes it simple to build and use kernels using modular device drivers.
Any module options specified in /etc/modules.conf are passed to the modules as they are loaded by the initial ramdisk.
If the root device is on a loop device (such as /dev/loop0), mkinitrd will build an initrd which sets up the loopback file properly. To do
this, the fstab must contain a comment of the form:
# LOOP0: /dev/hda1 vfat /linux/rootfs
LOOP0 must be the name of the loop device which needs to be configured, in all capital lettes. The parameters after the colon are the
device which contains the filesystem with the loopback image on it, the filesystem which is on the device, and the full path to the loop-
back image. If the filesystem is modular, initrd will automatically add the filesystem's modules to the initrd image.
The root filesystem used by the kernel is specified in the boot configuration file, as always. The traditional root=/dev/hda1 style device
specification is allowed. If a label is used, as in root=LABEL=rootPart the initrd will search all available devices for an ext2 or ext3
filesystem with the appropriate label, and mount that device as the root filesystem.
OPTIONS
--builtin=module
Act as if module is built into the kernel being used. mkinitrd will not look for this module, and will not emit an error if it does
not exist. This option may be used multiple times.
-f Allows mkinitrd to overwrite an existing image file.
--fstab=fstab
Use fstab to automatically determine what type of filesystem the root device is on. Normally, /etc/fstab is used.
--image-version
The kernel version number is appended to the initrd image path before the image is created.
--nocompress
Normally the created initrd image is compressed with gzip. If this option is specified, the compression is skipped.
--nopivot Do not use the pivot_root system call as part of the initrd. This lets mkinitrd build proper images for Linux 2.2 kernels
at the expense of some features. In particular, some filesystems (such as ext3) will not work properly and filesystem options will
not be used to mount root. This option is not recommended, and will be removed in future versions.
--omit-lvm-modules
Do not load any lvm modules, even if /etc/fstab expects them.
--omit-raid-modules
Do not load any raid modules, even if /etc/fstab and /etc/raidtab expect them.
--omit-scsi-modules
Do not load any scsi modules, including 'scsi_mod' and 'sd_mod' modules, even if they are present.
--preload=module
Load the module module in the initial ramdisk image. The module gets loaded before any SCSI modules which are specified in /etc/mod-
ules.conf. This option may be used as many times as necessary.
-v Prints out verbose information while creating the image (normally the mkinitrd runs silently).
--version
Prints the version of mkinitrd that's being used and then exits.
--with=module
Load the modules module in the initial ramdisk image. The module gets loaded after any SCSI modules which are specified in /etc/mod-
ules.conf. This option may be used as many times as necessary.
FILES
/dev/loop* A block loopback device is used to create the image, which makes this script useless on systems without block loopback
support available.
/etc/modules.conf Specified SCSI modules to be loaded and module options to be used.
SEE ALSO
fstab(5), insmod(1), kerneld(8), lilo(8)
AUTHOR
Erik Troan <ewt@redhat.com>
4th Berkeley Distribution Sat Mar 27 1999 MKINITRD(8)