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Full Discussion: iostat output vs TPC output (array layer)
Special Forums Hardware Filesystems, Disks and Memory iostat output vs TPC output (array layer) Post 302517362 by DGPickett on Tuesday 26th of April 2011 02:59:55 PM
Old 04-26-2011
Well, it is a complex world, with security and speed in opposition. One oddity of expanding sidk sizes is that one new big disk may be overwhelmed with the level of I/O that used to be handled by 8 disks, so size attracting query and churn is a negative! Striping allows the bandwidth of many drives to be applied to the combined storage, with supports more buffering with faster buffer fills, if things are sequential often enough. If everything was sequential and failure was no worry, you could stipe all together for max bandwidth, but you might do better with 2 or more virtual volumes so copying, database joining and such can be sequential on each virtual device. So, there are sometimes ways to force smart parallelism, the ability to join huge sets without seeks. However, RAM and 64 bit VM have made buffering so ample that it may dilute that sort of approach. RAID has not entirely freed us from failure worry, since with all the layers of software and hardware and vendors, it seems RAID errors often never get heard until they are 2 devices down. Rebuild time is not inconsequential, either. So, your approach should go beyond hot spots to maximizing the bandwidth of a managable number of virtual volumes. Along the way, look at the pathways and how they figure into the redundancy and striping. If a controller handles both sides of a mirror, and goes wonky . . . . If striping runs across all controllers, scsi cables, then any controller or cable bottleneck is diluted. Intellegent use of simple mirror for high churn and RAID-N for low churn is nice, too! Sometimes, this discussion can be extended down into the app, as DB2 append tables with insert never update or delete are churn free except at the end. Disk is cheap and 100% history is wise. Churn-free data might even migrate to some hierarchical read-only store like DVD arrays. Assuming control of chaos is someone else's job can be a luxury.

But, yeah, it seems like it is still good, but might not be sufficient, and an approach with sufficiency might not make it necessary.
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LEARN ABOUT CENTOS

ataraid

ATARAID(4)						   BSD Kernel Interfaces Manual 						ATARAID(4)

NAME

     ataraid -- ATA software RAID support

SYNOPSIS

     device ata
     device ataraid

DESCRIPTION

     The ataraid driver provides support for so-called software RAID (sometimes referred to as fake RAID or pseudo RAID).

     When a controller that supports software RAID is instructed to create a RAID array, its BIOS writes data structures in a specific metadata
     format to the disks.  These data structures are picked up by the ataraid driver, so that FreeBSD can work with the array.	The ataraid driver
     has to understand the specific metadata format of a controller BIOS in order to support its RAID capabilities.

     Read-only support for a metadata format means that FreeBSD can use the given RAID array for normal read/write operations.	Creation and
     rebuild of such arrays has to be done from the controller BIOS.

     Read and write support for a metadata format means that FreeBSD can use the given RAID array for normal read/write operations.  Additionally,
     the atacontrol(8) utility can be used to create, rebuild, update and fail such RAID arrays.

     The ataraid driver can read the following metadata formats:

     o	 Adaptec HostRAID
     o	 Highpoint V2 RocketRAID
     o	 Highpoint V3 RocketRAID
     o	 Intel MatrixRAID
     o	 Integrated Technology Express (ITE)
     o	 JMicron
     o	 LSI Logic V2 MegaRAID
     o	 LSI Logic V3 MegaRAID
     o	 NVIDIA MediaShield
     o	 Promise FastTrak
     o	 Silicon Image Medley
     o	 Silicon Integrated Systems (SiS)
     o	 VIA Tech V-RAID
     o	 FreeBSD PseudoRAID

     The ataraid driver can write the following metadata formats:

     o	 Highpoint V2 RocketRAID
     o	 Intel MatrixRAID
     o	 JMicron
     o	 Promise FastTrak
     o	 Silicon Integrated Systems (SiS)
     o	 VIA Tech V-RAID
     o	 FreeBSD PseudoRAID

     It is also possible to use software RAID on controllers that do not have special software RAID capabilities.  See atacontrol(8) for details.

FILES

     /dev/ar*  ATA RAID device nodes

SEE ALSO

     ata(4), atacontrol(8)

CAVEATS

     RAID5 is not supported at this time.  Code exists, but it neither uses nor maintains parity information.

AUTHORS

     The ataraid driver was written by Soren Schmidt <sos@FreeBSD.org>.  This manual page was written by Christian Brueffer
     <brueffer@FreeBSD.org>.

BSD
								 February 17, 2006							       BSD
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