raid-guestlec - Guest Lecture for 15-440 Disk Array Data...

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October 2010, Greg Ganger © 1 Guest Lecture for 15-440 Disk Array Data Organizations and RAID
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October 2010, Greg Ganger © 2 Plan for today Why have multiple disks? Storage capacity, performance capacity, reliability Load distribution problem and approaches disk striping Fault tolerance replication parity-based protection “RAID” and the Disk Array Matrix Rebuild
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October 2010, Greg Ganger © 3 Why multi-disk systems? A single storage device may not provide enough storage capacity, performance capacity, reliability So, what is the simplest arrangement?
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October 2010, Greg Ganger © 4 B0 B1 B2 B3 C0 C1 C2 C3 D0 D1 D2 D3 A0 A1 A2 A3 Just a bunch of disks (JBOD) Yes, it’s a goofy name industry really does sell “JBOD enclosures”
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October 2010, Greg Ganger © 5 Disk Subsystem Load Balancing I/O requests are almost never evenly distributed Some data is requested more than other data Depends on the apps, usage, time, …
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October 2010, Greg Ganger © 6 Disk Subsystem Load Balancing I/O requests are almost never evenly distributed Some data is requested more than other data Depends on the apps, usage, time, … What is the right data-to-disk assignment policy? Common approach: Fixed data placement Your data is on disk X, period! For good reasons too: you bought it or you’re paying more … Fancy: Dynamic data placement If some of your files are accessed a lot, the admin (or even system) may separate the “hot” files across multiple disks In this scenario, entire files systems (or even files) are manually moved by the system admin to specific disks
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October 2010, Greg Ganger © 7 Disk Subsystem Load Balancing I/O requests are almost never evenly distributed Some data is requested more than other data Depends on the apps, usage, time, … What is the right data-to-disk assignment policy? Common approach: Fixed data placement Your data is on disk X, period! Fancy: Dynamic data placement If some of your files are accessed a lot, we may separate the “hot” files across multiple disks In this scenario, entire files systems (or even files) are manually moved by the system admin to specific disks Alternative: Disk striping Stripe all of the data across all of the disks
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October 2010, Greg Ganger © 8 Disk Striping Interleave data across multiple disks Large file streaming can enjoy parallel transfers High throughput requests can enjoy thorough load balancing If blocks of hot files equally likely on all disks (really?) stripe unit or block Stripe File Foo:
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October 2010, Greg Ganger © 9 Disk striping details How disk striping works Break up total space into fixed-size stripe units Distribute the stripe units among disks in round-robin Compute location of block #B as follows disk# = B % N (%=modulo, N = # of disks) LBN# = B / N (computes the LBN on given disk)
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October 2010, Greg Ganger © 10 Now, What If A Disk Fails?
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