ApplicationProgrammer(e.g., business analyst, Data architect) Application SophisticatedApplicationProgrammer(e.g., SAP admin) QueryProcessor Indexes Storage Subsystem Concurrency Control Recovery DBA,Tuner Operating System Hardware[Processor(s), Disk(s), Memory]
Outline • Part 1: Tuning the storage subsystem • RAID storage system • Choosing a proper RAID level • Part 2: Enhancing the hardware configuration
1956: IBM (RAMAC) first disk drive 5 Mb – 0.002 Mb/in235000$/year9 Kb/sec 1980: SEAGATE first 5.25’’ disk drive 5 Mb – 1.96 Mb/in2625 Kb/sec 1999: IBM MICRODRIVE first 1’’ disk drive340Mb 6.1 MB/sec tracks spindle platter read/write head actuator disk arm Controller disk interface Magnetic Disks
Access Time (2001) Controller overhead (0.2 ms) Seek Time (4 to 9 ms) Rotational Delay (2 to 6 ms) Read/Write Time (10 to 500 KB/ms) Disk Interface IDE (16 bits, Ultra DMA - 25 MHz) SCSI: width (narrow 8 bits vs. wide 16 bits) - frequency (Ultra3 - 80 MHz). http://www.pcguide.com/ref/hdd/ Magnetic Disks
The familiar bandwidth pyramid: The farther from the CPU, the less the bandwidth. 40 133 422 15 per disk Hardware Bandwidth System Bandwidth Yesterday in megabytes per second (not to scale!) Slide courtesy of J. Gray/L.Chung Hard Disk | SCSI | PCI | Memory | Processor
The familiar pyramid is gone! PCI is now the bottleneck! In practice, 3 disks can reach saturation using sequential IO 26 26 160 133 1,600 26 Hardware Bandwidth System Bandwidth Today in megabytes per second (not to scale!) Slide courtesy of J. Gray/L.Chung Hard Disk | SCSI | PCI | Memory | Processor
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
RAID 0 - Striping • No redundancy • No fault tolerance • High I/O performance • Parallel I/O
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
RAID 3 - Parallel Array with Parity • Fast read/write • All disk arms are synchronized • Speed is limited by the slowest disk
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
RAID 5 – Parity Checking • For error detection, rather than full redundancy • Each stripe unit has an extra parity stripe • Parity stripes are distributed
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).
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
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
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.
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.
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. RAID Levels
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.
SCSI Controller Cache - Data Settings: employees(ssnum, name, lat, long, hundreds1, hundreds2); create clustered index c on employees(hundreds2); • Employees table partitioned over two disks; Log on a separate disk; same controller (same channel). • 200 000 rows per table • Database buffer size limited to 400 Mb. • Dual Xeon (550MHz,512Kb), 1Gb RAM, Internal RAID controller from Adaptec (80Mb), 4x18Gb drives (10000RPM), Windows 2000.
SCSI (not disk) Controller Cache - Transactions No Concurrent Transactions: update employees set lat = long, long = lat where hundreds2 = ?; • cache friendly: update of 20,000 rows (~90Mb) • cache unfriendly: update of 200,000 rows (~900Mb)
SQL Server 7 on Windows 2000. Adaptec ServerRaid controller: 80 Mb RAM Write-back mode Updates Controller cache increases throughput whether operation is cache friendly or not. Efficient replacement policy! SCSI Controller Cache
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.
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
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
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
Add Processors • Function parallelism • Use different processors for different tasks • GUI, Query Optimisation, TT&CC, different types of apps, different users • Operation pipelines: • E.g., scan, sort, select, join… • Easy for RO apps, hard for update apps • Data partition parallelism • Partition data, thus the operation on the data
Parallelism • Some tasks are easier to parallelize • E.g., join phase of GRACE hash join • E.g., scan, join, sum, min • Some tasks are not so easy • E.g., sorting, avg, nested-queries
Summary • We have covered: • The storage subsystem • RAID: what are they and which one to use? • Memory, disks and processors • When to add what?
Database Tuning Database Tuning is the activity of making a database application run more quickly. “More quickly” usually means higher throughput, though it may mean lower response time for time-critical applications.
Tuning Principles • Think globally, fix locally • Partitioning breaks bottlenecks (temporal and spatial) • Start-up costs are high; running costs are low • Render onto server what is due onto Server • Be prepared for trade-offs (indexes and inserts)
Tuning Mindset • Set reasonable performance tuning goals • Measure and document current performance • Identify current system performance bottleneck • Identify current OS bottleneck • Tune the required components eg: application, DB, I/O, contention, OS etc • Track and exercise change-control procedures • Measure and document current performance • Repeat step 3 through 7 until the goal is met
Goals Met? • Appreciation of DBMS architecture • Study the effect of various components on the performance of the systems • Tuning principle • Troubleshooting techniques for chasing down performance problems • Hands-on experience in Tuning