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《MicrosoftSQLServerDatabaseEngineIO》.docx
Microsoft SQL ServerDatabase Engine I/O
by Bob Dorr, Microsoft SQL Server Principle Escalation Engineer, 1994 – PresentBuilt: Jan 2008
Areas Covered
Write Ahead Logging (WAL) Protocol
Synchronous vs Asynchronous I/O
Scatter / Gather I/O
Sector alignment, Block Alignment
Latching and a page: A read walk-through
SQL Server I/O Sizes
Data cache maintenance
PAE and AWE
Read Ahead
User Mode and Kernel Mode (SYSTRAP)
Sparse Files and Copy On Write (COW) Pages
Locked Pages
Scribbler(s) and Bit flips
Page Protection and Constant Pages
Checksum vs Torn
Stale Read
Stalled I/O
WAL Protocol
Write Ahead Logging
ACID (Durability Property)
Log records secured before data
Hardened / Stable Media
Log contains parity bit
Commit
Rollback
Trigger Snapshot
Synchronous vs Asynchronous I/O
Sync: Wait for Completion
Async: Post and Continue
Overlapped
Event
Completion Port
SQL Server
98% Async Usage
Overlapped and HasOverlappedIoCompleted
Network Layers Use Completion Port
Backup/Restore Use Sync – Sequential Patterns
dm_io_pending_io_requests
Overlapped Structure
Async Processing ~= CPU
Package vs Phone
Scatter / Gather I/O
Consolidates or Distributes
APIs
ReadFileScatter
WriteFileGather
Increases Efficiency
Used by SQL I/O Paths
Used by Windows Page File
Old Design: 6.x Sorting
AWE Availability
WriteMultiple
# of 8K Pages
Forward and Backward
Buffer Pool Ramp-up
Disk
Memory
Gather
Scatter
Sector AlignmentBlock Alignment
Sector: Log Writes
Block: Performance
Avoid Crossovers
DiskPart/DiskPar Utilities
Discuss with your Vendor
Double Touch
Rewrites
Defragment
4K Sectors
Alignment: /kb/929491
To verify that an existing partition is aligned, divide the size of the stripe unit by the starting offset of the RAID disk group. Use the following syntax: ((Partition offset) * (Disk sector size)) / (Stripe unit size)
Example of alignment calculations in bytes for a 256-KB stripe unit size: (63 * 512) / 262144 = 0.123046875(64 * 512) / 262144 = 0.125(128 * 512) /
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