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RAID General Concept

RAID General Concept. Auteur : Franck THOMAS. RAID Basics. RAID : Redundant Arrays of Independent (or Inexpensive) Disks Definition : Simultaneous use of two or more drives in order to add fault tolerance, capacity and/or performance to data storage system. HDD Performance.

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RAID General Concept

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  1. RAID General Concept Auteur : Franck THOMAS NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  2. RAID Basics • RAID : Redundant Arrays of Independent (or Inexpensive) Disks • Definition : Simultaneous use of two or more drives in order to add fault tolerance, capacity and/or performance to data storage system NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  3. HDD Performance • Technologies offer different price and performances • Specification with impact on performances: rotation speed, cache size, cache management (NCQ), port SCSI hard disk are no more used in servers or workstations since end of 2008. They have been replaced by SAS HDD. NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  4. RAID 0 controller • RAID 0 (stripping): • Data are stripped on all disks • Offer performances • No redundancy • 2 disks minimum, maximum depending of RAID controller Data are split depending of stripe size (16/32/64/128KB) With software RAID, there is also concatenation / spanning mode NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  5. JBOD controller JBOD Disk “JBOD”: Just a Bunch of Disk A JBOD disk is physically connected to RAID controller but doesn’t use the RAID functionalities. This disk is usable as if it was connected to simple SCSI controller The goal is to have RAID drives and non RAID drives into the same system. NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  6. RAID 1 controller • RAID 1 (mirroring): • Data duplicated on second hard disk • Offer redundancy • Equivalent of one disk space lost for redundancy • Only on 2 disks • Support one disk failure NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  7. RAID 10 controller • RAID 10 (stripping+mirroring): • Aggregation of several mirrors • Offer redundancy • Offer performance • Half of physical space lost for duplication • Even amount of disk required (4 minimum) • Support one disk failure per mirror NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  8. RAID 5 controller • RAID 5 (stripping with parity): • Data stripped on all disks • Redundancy done by parity (XOR logical operator) • Parity distributed on all disks • Equivalent of one disk space is used for parity storage (1/n disk lost) • 3 disks minimum, maximum given by controller • Support one disk failure NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  9. RAID 6 – Triple Mirror controller • RAID 6-TM: • Data mirrored on 3 disks • Up to 2 disks lost • Equivalent of 2 disks space used redundancy • 3 disks minimum and maximum NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  10. RAID 6 – Double Parity • RAID 6 (stripping with duplicated parity): • Data stripped on all disks • Redundancy done by parity (XOR logical operator) • Parity splitted and duplicated on all disks alternatively • Equivalent of 2 disks space is used for parity storage (2/n disk lost) • 4 disks minimum, maximum given by controller • Support 2 disk failures NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  11. Non exhaustive list of «exotic / obsolete» RAID levels • RAID 0+1: A mirror of 2 RAID 0 • RAID 1E: A RAID 0 where stripes are written twice and distributed across several disks = RAID1 on odd amount of disks. • RAID 3: RAID 5 where a single disk is dedicated to parity storage • RAID 5E, 5EE: Specific from LSI • RAID 7: RAID 0 using concatenation mode (with hdd of different sizes) • RAID 50: stripping of several RAID5 • RAID 60: stripping of several RAID6 • … NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  12. Expansion / Migration • Expansion: Possibility to expand size of RAID array by adding a disk.e.g. Expand a RAID5 with a new disk. • Migration: Possibility to change RAID level, eventually by adding disk. e.g. Migrate from RAID 1 to RAID 0 (still 2 disks). Always backup data as precaution but operation doesn’t impact data nor access to them. Some controllers offer to migrate ‘ONLINE’ NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  13. RAID Signature – COD (Conf. On Disk) • RAID configuration is always written on disks. • Signature is around 700 MB big. • Today, most of RAID controllers doesn’t contain any configuration to avoid configuration mismatch. Some old controllers (LSI SCSI) stored RAID configuration, so be careful on RAID card swap ! • Plug controller on server without any disk connected • Start server and « Clear configuration », stop server • Replug disk and start server, controller will load configuration automatically NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  14. RAID and operating system NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  15. Size of Physical Drive under Operating System Example two HDD of 300GB in RAID 1 will provide a size of less than 286GB under OS. 300.000.000 / 1024 / 1024 = 286,102 GB 286,102 – COD (≈ 700MB) = 285,402 GB RAID 1 300 GB COD COD NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  16. Limitations on disk size (Windows) • A disk signed as ‘MBR’ type is limited to 2 TB. • A disk where Windows is installed is always ‘MBR’ type. • The only way to access more than 2 TB is to create a 2nd LD and convert it as ‘GPT’ Note: ‘GPT’ is available since Windows Server 2003 SP1 or more. More information on the following link : http://technet2.microsoft.com/WindowsServer/en/library/4b35160a-4e27-4258-9e8b-e2088f8a757a1033.mspx NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  17. Status: Online or Unconfigured controller Array OPTIMAL or ONLINE ONLINE ONLINE READY or UNCONFIGURED • Disk “ONLINE”: • ONLINE disk is a physical disk used or integrated in an Array. • If all disks of an array are ONLINE, array status is ONLINE or OPTIMAL. • Disk “READY or UNCONFIGURED”: • Physical disk not used by the controller. Can be removed without impact NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  18. Status: Offline / Dead or Failed controller controller OFFLINE or FAILED ONLINE ONLINE Array A0 DEGRADED or CRITICAL DEAD or FAILED (not responding) ONLINE ONLINE • Disk “OFFLINE or FAILED”: • Such disk is still in the array but inactive. • The array is now DEGRADED or CRITICAL. • It can be a minor error or a status manually forced by administrator. • REBUILD required to reintegrate the disk in the array. • Disk “DEAD or FAILED – NOT RESPONDING”: • Like the OFFLINE status but means this is an hardware failure regarding the detection of the disk NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  19. Status: Offline / Dead or Failed controller controller OFFLINE or FAILED OFFLINE or FAILED OFFLINE or FAILED Array A0 OFFLINE or FAILED DEAD or FAILED (not responding) DEAD or FAILED (not responding) ONLINE • Array “OFFLINE”: • All disks are OFFLINE. All data could be lost, but you can try to force all disks in ONLINE to retrieve the original configuration and data. • Array “FAILED”: • Two or more disks are OFFLINE but not all disks of the array. • All the data could be lost, but you can try to force disk in ONLINE to retrieve the original configuration and data. NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  20. Status: Hot Spare & Rebuild FAILURE controller Array Disk FAILED or OFFLINE or DEAD REBUILD Segment 1 duplicated Segment 2 duplicated Segment 3 duplicated Segment 4 duplicated controller Array HOT SPARE Disk “Hot Spare”: Hot spare disk is a standby disk ready to replace a failing (Offline or Dead) drive automatically This disk is not used until a failure occurs. After the rebuild, this disk is part of the array NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  21. Initialization • Preparing a physical drive is called : format. • Preparing a logical drive is called: initialisation. • Initialisation will erase all sectors of logical drive. • Two modes exists: • Full Initialisation: all blocks of logical drive are erased, longer but safer • Quick Initialisation: only first blocks of logical drive are erased and remaining block will be erased in background, shorter. NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  22. Patrol Read / Media Patrol • In order to detect bad sector independently of normal I/O activity, some controllers offer sector verification in background when system is idle. • According to controller, this feature is called “Patrol read” or “Media Patrol” • It can be done on non RAID drives (JBOD, Spare) and HDD in RAID. • If bad sector is detected, controller will notify of error • If HDD is not in RAID, data recovery cannot be applied. NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  23. Consistency Check Example here with 36GB RAID1 made of 2 x 15kRPM SAS disk linked to LSI 8408E. • Consistency Check is insuring that data are readable and redundant • It applies on logical drive level with RAID level offering redundancy. • It is a preventive maintenance task to be scheduled monthly. • In case of inconsistency (ex: bad sector), the sector is dynamically remapped using HDD spare sectors and date are recovered from rest of RAID. NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  24. Performance • 2 families: • « Software » = Disk Controller +Software • Software part is done by ROM BIOS and driver. • Workload is on server CPU. • No memory cache, nor BBU. • It is also known as « HostRAID » • Solution integrated on motherboard so cheapest solution • « Hardware » = • Disk Controller + RAID Engine • Dedicated controller, no CPU load. • Memory cache / BBU • Solution on daughter PCI card Disk controller HDD SouthBridge PCI HDD RAID Engine Disk Controller Internal bus SouthBridge PCI HDD HDD NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  25. Cache memory • In order to optimise physical access to hard disk, some RAID controllers offer cache memory (option or on board) • Cache Memory is always used for read access. • Cache Memory may be used for write access: • For write access, two mode exists: • Write through = write cache disabled • Write back = write enabled • Write back is risky because data are not immediately written on disk. If power failure occurs, data may be lost. NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  26. BBU Battery Back Up Unit (BBU) option is designed to add fault tolerance against power failure. The BBU powers the memory until electricity comes back When BBU is present, write cache can be set to Write Back Ex1: LSI SecuRAID321 Ex2: Promise FastTrak S150 SX4 PCI During maintenance operation, make sure to unplug battery before memory removal NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  27. Performance vs. RAID Levels and technology RAID 0 RAID 5 MB/sec NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

  28. Questions ? NEC Computers SAS - Confidential - Oct 2008 - RAID General Concept

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