3. Storage systems.pdf - Storage Systems INF 551 Wensheng Wu 1 Storage hierarchy Speed Secondary(Hard disks SSD Tertiary(Tape Optical Disk Cost Energy

3. Storage systems.pdf - Storage Systems INF 551 Wensheng...

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Storage Systems 1 INF 551 Wensheng Wu
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Storage hierarchy 2 Primary (Cache, Memory) Secondary (Hard disks, SSD) Tertiary (Tape, Optical Disk) Speed Cost, Energy Consumption
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Access times LEVEL ACCESS TIME TYPICAL SIZE Registers "instantaneous" under 1KB Level 1 Cache 1-3 ns 64KB per core Level 2 Cache 3-10 ns 256KB per core Level 3 Cache 10-20 ns 2-20 MB per chip Main Memory 30-60 ns 4-32 GB per system Hard Disk 3,000,000-10,000,000 ns over 1TB 3 Resource: - technology/2012/06/inside-the-ssd-revolution-how- solid-state-disks-really-work/ Ssd: 25,000 ns
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CRUD Basic functions of a storage device CRUD: (C)reate/write (R)ead (U)pdate/overwrite (D)elete 4
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Characterizing a storage device Capacity (bytes) How much data it can hold Cost ($$$) Price per byte of storage Bandwidth (bytes/sec) Number of bytes that can be transferred per second Note that read and write bandwidth may be different Latency (secs) Time between initiating a request and an action In the case of storage, action is to deliver 1 st Byte 5
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Time to complete an operation Time to complete an operation depends on both bandwidth and latency CompletionTime = Latency + Size/Bandwidth The time of a workload will depend on Technology, e.g., hard drive/ssd Operation type, e.g., read/write Number of operations in the workload Access pattern (random vs. sequential) 6
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Access pattern Sequential Data to be accessed are located next to each other or sequentially on the device Random Access data located randomly on storage device 7
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Magnetic recording Write head Applies electrical current to write head Changes direction of magnetic field under head 8
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Reading Read head senses direction of magnetic field 9
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Road map Tapes Hard disk Solid state drive 10
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Linear tape Data recorded on parallel tracks that span the length of the tape 11
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Tapes Current technology is LTO Linear Tape-Open (an open standard) Characteristics Capacity up to 6.25 TB per tape (LTO-7) Drive cost ~ $2500 Tape cost ~ $45 for 2.5TB tape Tape access time (~ minute) Time to mount the tape Time to wind the tape to correct position Data transmission rates ~ 250MB/sec 12 $200 300MB/s LTO-7
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Performance characteristics High latency/low cost makes tape most appropriate for "archival" storage Low frequency of reads Very large data objects Random access will be slow due to latency Sequential reads will be fast 13
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Linear tape file system Two partitions on tape First contains metadata and directories. Tape reader can find and load this very quickly Second contains blocks for data Directory structure coded in XML Self describing file format… 14
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Tape Cartridge 16
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17 12.8TB 6.25TB
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A tape library 18
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Inside a robotic tape library 778 19
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Road map Tapes Hard disk Solid state drive 20
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