RAID and Other Disk DetailsReview: Magnetic Disk CharacteristicReview: Disk SchedulingGoals for TodayHow is the disk formatted?Cylinder SkewFormatting and PerformanceDisk PartitioningHandling ErrorsRAID MotivationRAIDSome Raid IssuesRaid Level 0Raid Level 1Parity and Hamming CodeSlide 16Raid Level 2Raid Level 3Raid Level 4Raid Level 5Raid Level 6RAID 0+1 and 1+0Stable StorageApproachCD-ROMsSlide 26AnnouncementsGrade distributionPowerPoint PresentationRAID and Other Disk Details2Review: Magnetic Disk Characteristic•Cylinder: all the tracks under the head at a given point on all surface•Read/write data is a three-stage process:–Seek time: position the head/arm over the proper track (into proper cylinder)–Rotational latency: wait for the desired sectorto rotate under the read/write head–Transfer time: transfer a block of bits (sector)under the read-write head•Disk Latency = Queueing Time + Controller time +Seek Time + Rotation Time + Xfer Time•Highest Bandwidth: –transfer large group of blocks sequentially from one trackSectorTrackCylinderHeadPlatterSoftwareQueue(Device Driver)HardwareController Media Time(Seek+Rot+Xfer)RequestResult3Review: Disk Scheduling•Disk can do only one request at a time; What order do you choose to do queued requests?•FIFO Order–Fair among requesters, but order of arrival may be to random spots on the disk Very long seeks•SSTF: Shortest seek time first–Pick the request that’s closest on the disk–Although called SSTF, today must include rotational delay in calculation, since rotation can be as long as seek–Con: SSTF good at reducing seeks, but may lead to starvation•SCAN: Implements an Elevator Algorithm: take the closest request in the direction of travel–No starvation, but retains flavor of SSTF•C-SCAN: Circular-Scan: only goes in one direction–Skips any requests on the way back–Fairer than SCAN, not biased towards pages in middle•LOOK/C-LOOK similar to SCAN/C-SCAN, but skips end of disk2,32,13,107,25,22,2HeadUserRequests142Disk Head34Goals for Today•Finish discussion on disk formatting•How to tolerate disk failure?•Prelim graded!5How is the disk formatted?•After manufacturing disk has no information–Is stack of platters coated with magnetizable metal oxide•Before use, each platter receives low-level format–Format has series of concentric tracks–Each track contains some sectors–There is a short gap between sectors•Preamble allows h/w to recognize start of sector–Also contains cylinder and sector numbers–Data is usually 512 bytes–ECC field used to detect and recover from read errors6Cylinder Skew•Why cylinder skew?•How much skew?•Example, if–10000 rpm•Drive rotates in 6 ms–Track has 300 sectors•New sector every 20 µs–If track seek time 800 µs40 sectors pass on seekCylinder skew: 40 sectors7Formatting and Performance•If 10K rpm, 300 sectors of 512 bytes per track–153,600 bytes every 6 ms 24.4 MB/sec transfer rate•If disk controller buffer can store only one sector–For 2 consecutive reads, 2nd sector flies past during memory transfer of 1st track–Idea: Use single/double interleaving8Disk Partitioning•Each partition is like a separate disk•Sector 0 is MBR –Contains boot code + partition table–Partition table has starting sector and size of each partition•High-level formatting–Done for each partition–Specifies boot block, free list, root directory, empty file system•What happens on boot?–BIOS loads MBR, boot program checks to see active partition–Reads boot sector from that partition that then loads OS kernel, etc.9Handling Errors•A disk track with a bad sector•Solutions:–Substitute a spare for the bad sector (sector sparing)–Shift all sectors to bypass bad one (sector forwarding)10RAID Motivation•Disks are improving, but not as fast as CPUs–1970s seek time: 50-100 ms.–2000s seek time: <5 ms.–Factor of 20 improvement in 3 decades•We can use multiple disks for improving performance–By Striping files across multiple disks (placing parts of each file on a different disk), parallel I/O can improve access time•Striping reduces reliability –100 disks have 1/100th mean time between failures of one disk•So, we need Striping for performance, but we need something to help with reliability / availability•To improve reliability, we can add redundant data to the disks, in addition to Striping11RAID•A RAID is a Redundant Array of Inexpensive Disks–In industry, “I” is for “Independent”–The alternative is SLED, single large expensive disk•Disks are small and cheap, so it’s easy to put lots of disks (10s to 100s) in one box for increased storage, performance, and availability•The RAID box with a RAID controller looks just like a SLED to the computer•Data plus some redundant information is Striped across the disks in some way•How that Striping is done is key to performance and reliability.12Some Raid Issues•Granularity–fine-grained: Stripe each file over all disks. This gives high throughput for the file, but limits to transfer of 1 file at a time –coarse-grained: Stripe each file over only a few disks. This limits throughput for 1 file but allows more parallel file access•Redundancy–uniformly distribute redundancy info on disks: avoids load-balancing problems –concentrate redundancy info on a small number of disks: partition the set into data disks and redundant disks13Raid Level 0•Level 0 is nonredundant disk array•Files are Striped across disks, no redundant info•High read throughput•Best write throughput (no redundant info to write)•Any disk failure results in data loss–Reliability worse than SLEDStripe 0Stripe 4Stripe 3Stripe 1 Stripe 2Stripe 8Stripe 10Stripe 11Stripe 7Stripe 6Stripe 5Stripe 9data disks14Raid Level 1•Mirrored Disks•Data is written to two places –On failure, just use surviving disk•On read, choose fastest to read –Write performance is same as single drive, read performance is 2x better•Expensivedata disks mirror copiesStripe 0Stripe 4Stripe 3Stripe 1 Stripe 2Stripe 8Stripe 10Stripe 11Stripe 7Stripe 6Stripe 5Stripe 9Stripe 0Stripe 4Stripe 3Stripe 1 Stripe 2Stripe 8Stripe 10Stripe 11Stripe 7Stripe 6Stripe 5Stripe 915Parity and Hamming Code•What do you need to do in order to detect and correct a one-bit error ?–Suppose you have a binary number, represented as a collection of bits: <b3, b2, b1, b0>, e.g. 0110•Detection is
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