W4118: RAID Instructor: Junfeng Yang References: Modern Operating Systems (3rd edition), Operating Systems Concepts (8th edition), previous W4118, and OS at MIT, Stanford, and UWisc RAID motivation ? Performance ? Disks are slow compared to CPU ? Disk speed improves slowly compared to CPU ? Reliability ? In single disk systems, one disk failure ? data loss ? Cost ? A single fast, reliable disk is expensive 1 RAID idea ? RAID idea: use redundancy to improve performance and reliability ? Redundant array of cheap disks as one storage unit ? Fast: simultaneous read and write disks in the array ? Reliable: use parity to detect and correct errors ? RAID can have different redundancy levels, achieving different performance and reliability ? Seven different RAID levels (0-6) 2 Evaluating RAID ? Cost ? Storage utilization: data capacity / total capacity ? Reliability ? Tolerance of disk failures ? Performance ? (Large) sequential read, write, read-modify-write ? (Small) random read, write, read-modify-write ? Speedup over a single disk 3 Computing cost ? G = number of data disks in a RAID group ? C = number of check disks in a RAID group ? Cost = C/(G+C) 4 Computing reliability ? N = total number of disks ? G = number of data disks in a RAID group ? C = number of check/parity disks in a RAID group ? MTTFdisk = mean time to failure for a disk ? MTTR = mean time to repair for a failed disk ? MTTFraid = ? 5 RAID 0: non-redundant striping ? Structure ? Data striped across all disks in an array ? No parity ? Advantages: ? Good performance: with N disks, roughly N times speedup ? Disadvantages: ? Poor reliability: one disk failure ? data loss 6 RAID 0 performance 7 RAID 1: mirroring ? Structure ? Keep a mirrored (shadow) copy of data ? Advantages ? Good reliability: one disk failure OK ? Good read performance ? Disadvantage ? High cost: one data disk requires one parity disk 8 RAID 1 performance 9 RAID 2: error-correction parity ? Structure ? A data sector striped across data disks ? Compute error-correcting parity and store in parity disks ? Advantages ? Good reliability with higher storage utilization than mirroring ? Disadvantages ? Unnecessary cost: disk can already detect failure ? Poor random performance 10 parity disks RAID 3: bit-interleaved parity ? Structure ? Single parity disk (XOR of each stripe of a data sector) ? Advantages ? Same reliability with one disk failure as RAID2 since disk controller can determine what disk fails ? Higher storage utilization ? Disadvantages ? Poor random performance 11 RAID 4: block-interleaved parity ? Structure ? A set of data sectors (parity group) striped across data disks ? Advantages ? Same reliability as RAID3 ? Good random read performance ? Disadvantages ? Poor random write and read-modify-write performance 12 P RAID 4 performance 13 RAID 5: block-interleaved distributed parity ? Structure ? Parity sectors distributed across all disks ? Advantages ? Good performance 14 P P P P RAID 5 performance 15 RAID6: P+Q redundancy ? Structure ? Same as RAID 5 except using two parity sectors per parity group ? Advantages ? Can tolerate two disk failures 16 P Q P Q P Q Q P RAID levels PowerPoint Presentation
STUDYBLUE makes things that make you better at school.
Things like
online flashcards with photos and audio.
Things like personalized quizzes and friendly reminders about when (and what) to study next.
Think of it as a digital backpack™: access to all of your study materials online and on your phone.
STUDYBLUE exists to make studying efficient and effective for every student, for free.
Join us.
“I have used this website for three exams, and I see a huge difference in my test results.”
Naj