Transcript Document
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Magnetic Disks
tracks Controller spindle read/write head 1956: IBM (RAMAC) first disk drive 5 Mb – 0.002 Mb/in2 35000$/year 9 Kb/sec 1980: SEAGATE disk arm disk interface first 5.25’’ disk drive 5 Mb – 1.96 Mb/in2 625 Kb/sec 1999: IBM MICRODRIVE first 1’’ disk drive 340Mb 6.1 MB/sec 1
Hardware Bandwidth System Bandwidth Yesterday
in megabytes per second (not to scale!)
The familiar bandwidth pyramid: 15 per disk 40 133 422 The farther from the CPU, the less the bandwidth.
Slide courtesy of J. Gray/L.Chung
Hard Disk | SCSI | PCI | Memory | Processor 2
Hardware Bandwidth System Bandwidth Today
in megabytes per second (not to scale!)
Slide courtesy of J. Gray/L.Chung
26 The familiar pyramid is gone!
PCI is now the bottleneck!
26 160 133 1,600 26 In practice, 3 disks can reach saturation using sequential IO Hard Disk | SCSI | PCI | Memory | Processor 3
RAID Storage System
Redundant Array of Inexpensive Disks
Combine multiple small, inexpensive disk drives into a group to yield performance exceeding that of one large, more expensive drive Appear to the computer as a single virtual drive Support fault-tolerance by redundantly storing information in various ways 4
RAID 0 - Striping
No redundancy No fault tolerance High I/O performance Parallel I/O 6
RAID 1 – Mirroring
Provide good fault tolerance Works ok if one disk in a pair is down One write = a physical write on each disk One read = either read both or read the less busy one Could double the read rate 7
RAID 3 - Parallel Array with Parity
Fast read/write All disk arms are synchronized Speed is limited by the slowest disk 8
Parity Check - Classical
An extra bit added to a byte to detect errors in storage or transmission Even (odd) parity means that the parity bit is set so that there are an even (odd) number of one bits in the word, including the parity bit A single parity bit can only detect single bit errors since if an even number of bits are wrong then the parity bit will not change It is not possible to tell which bit is wrong 9
RAID 5 – Parity Checking
For error detection, rather than full redundancy Each stripe unit has an extra parity stripe Parity stripes are distributed 10
RAID 5 Read/Write
Read: parallel stripes read from multiple disks Good performance Write: 2 reads + 2 writes Read old data stripe; read parity stripe (2 reads) XOR old data stripe with new data stripe.
XOR result into parity stripe.
Write new data stripe and new parity stripe (2 writes).
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RAID 10 – Striped Mirroring
RAID 10 = Striping + mirroring A striped array of RAID 1 arrays High performance of RAID 0, and high tolerance of RAID 1 (at the cots of doubling disks) .. More information about RAID disks at http://www.acnc.com/04_01_05.html
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Hardware vs. Software RAID
Software RAID Software RAID: run on the server’s CPU Directly dependent on server CPU performance and load Occupies host system memory and CPU operation, degrading server performance Hardware RAID Hardware RAID: run on the RAID controller’s CPU Does not occupy any host system memory. Is not operating system dependent Host CPU can execute applications while the array adapter's processor simultaneously executes array functions: true hardware multi-tasking 13
RAID Levels - Data Settings:
accounts
( number, branchnum, balance); create clustered index c on accounts(number); 100000 rows Cold Buffer Dual Xeon (550MHz,512Kb), 1Gb RAM, Internal RAID controller from Adaptec (80Mb), 4x18Gb drives (10000RPM), Windows 2000.
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RAID Levels - Transactions
No Concurrent Transactions: Read Intensive: select avg(balance) from accounts; Write Intensive, e.g. typical insert: insert into accounts values (690466,6840,2272.76); Writes are uniformly distributed.
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RAID Levels
Read-Intensive 160 120 80 40 0 80000 60000 40000 20000 0 Soft RAID5 RAID5 RAID0 RAID10 RAID1 Single Disk Write-Intensive Soft RAID5 RAID5 RAID0 RAID10 RAID1 Single Disk
SQL Server7 on Windows 2000 (SoftRAID means striping/parity at host) Read-Intensive: Using multiple disks (RAID0, RAID 10, RAID5) increases throughput significantly. Write-Intensive: Without cache, RAID 5 suffers. With cache, it is ok.
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Comparing RAID Levels
RAID 0 RAID 1 RAID 5 Read Write Fault tolerance Disk utilization Key problems Key advantages
High High No High 2X 1X Yes Low High Medium Yes High Data lost when any disk fails Use double the disk space High I/O performance Very high I/O performance Lower throughput with disk failure A good overall balance
RAID 10
High High Yes Low Very expensive, not scalable High reliability with good performance 17
Controller Pre-fetching No, Write-back Yes
Read-ahead: Prefetching at the disk controller level. No information on access pattern.
Better to let database management system do it. Write-back vs. write through: Write back: transfer terminated as soon as data is written to cache. Batteries to guarantee write back in case of power failure Write through: transfer terminated as soon as data is written to disk.
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Which RAID Level to Use?
Data and Index Files RAID 5 is best suited for read intensive apps or if the RAID controller cache is effective enough.
RAID 10 is best suited for write intensive apps.
Log File RAID 1 is appropriate Fault tolerance with high write throughput. Writes are synchronous and sequential. No benefits in striping.
Temporary Files RAID 0 is appropriate. No fault tolerance. High throughput.
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What RAID Provides
Fault tolerance It does not prevent disk drive failures It enables real-time data recovery High I/O performance Mass data capacity Configuration flexibility Lower protected storage costs Easy maintenance 20
Enhancing Hardware Config.
Add memory
Cheapest option to get better performance Can be used to enlarge DB buffer pool Better hit ratio If used for enlarge OS buffer (as disk cache), it benefits but to other apps as well
Add disks Add processors
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Add Disks
Larger disk ≠better performance Bottleneck is disk bandwidth Add disks for A dedicated disk for the log Switch RAID5 to RAID10 for update-intensive apps Move secondary indexes to another disk for write intensive apps Partition read-intensive tables across many disks Consider intelligent disk systems Automatic replication and load balancing 22