Transactions and Reliability

File system components

Disk managementNamingReliability

What are the reliability issues in file systems?

Security

File system reliability issues

Heavy caching to achieve higher performance. Machines crash all the time. How to maintain a

consistent file system after the machine crashes?

Disk can crash. How to keep the data even when the disk breaks?

Caching issue

File systems have lots of metadata:Free blocks, directories, file headers, indirect

blocks

Metadata is heavily cached for performanceModification to the metadata may not be on

disk right away.

Problem

System crashes: all cached data are lost!!OS needs to ensure that the file system does

not reach an inconsistent state

Example: move a file between directoriesRemove a file from the old directoryAdd a file to the new directory

What happens when a crash occurs in the middle?

UNIX File System (Ad Hoc Failure-Recovery)

Metadata handling:Uses a synchronous write-through caching

policyA call to update metadata does not return until the

changes are propagated to disk

Updates are orderedWhen crashes occur, run fsck to repair in-

progress operations

Some Examples of Metadata Handling

Undo effects not yet visible to usersIf a new file is created, but not yet added to the

directory--Delete the file

Continue effects that are visible to usersIf file blocks are already allocated, but not

recorded in the bitmap--Update the bitmap

UFS User Data Handling

Uses a write-back policyModified blocks are written to disk at 30-second

intervals--Unless a user issues the sync system call

Data updates are not orderedIn many cases, consistent metadata is good

enough

Current solution: the Transaction Approach (Journaling)A transaction groups operations as a

unit, with the following characteristics:Atomic: all operations in a transaction either

happen or they do not (no partial operations)Serializable: transactions appear to happen

one after the otherDurable: once a transaction happens, it is

recoverable and can survive crashes

More on Transactions

A transaction is not done until it is committed

Once committed, a transaction is durableIf a transaction fails to complete, it must

rollback as if it did not happen at all

Transaction Implementation (One Thread)

Example: money transfer

Begin transaction

x = x – $100;

y = y + $100;

Commit

Transaction Implementation (One Thread)

Common implementations involve the use of a log, a journal that is never erased

A file system uses a write-ahead log to track all transactions information is written to the log before written

into the disk.

Transaction Implementation (One Thread)

Once accounts of x and y are on a log, the log is committed to disk in a single write

Actual changes to those accounts are done later

Transaction Illustrated

x = 1;y = 1;

x = 1;y = 1;

Transaction Illustrated

x = 1;y = 1;

x = 0;y = 2;

Transaction Illustrated

x = 1;y = 1;

x = 0;y = 2;

begin transaction

old x: 1

old y: 1

new x: 0

new y: 2

commit

Commit the log to disk before updating the actual values on disk

Transaction Steps

Mark the beginning of the transactionLog the changes in account xLog the changes in account yCommitModify account x on diskModify account y on disk

Scenarios of Crashes

If a crash occurs after the commitReplays the log to update accounts

If a crash occurs before the commitRolls back and discard the transaction

A crash cannot occur during the commitCommit is built as an atomic operatione.g. writing a single sector on disk

Two-Phase Locking (Multiple Threads)

Logging alone not enough to prevent multiple transactions from trashing one another (not serializable)

Solution: two-phase locking1. Acquire all locks

2. Perform updates and release all locks

Thread A cannot see thread B’s changes until thread B commits and releases locks

Transactions in File Systems

Many recent file systems built use write-ahead loggingWindows NT, Solaris, ext3 (Linux), etc

+ Eliminates running fsck after a crash

+ Write-ahead logging provides reliability

- All modifications need to be written twice

Log-Structured File System (LFS)

If logging is so great, why don’t we treat everything as log entries?

Log-structured file systemEverything is a log entry (file headers,

directories, data blocks)Write the log only once

Use version stamps to distinguish between old and new entries

More on LFS

New log entries are always appended to the end of the existing logAll writes are sequentialSeeks only occurs during reads

Not so bad due to temporal locality and caching

Problem:Need to create more contiguous space all the

time

RAID: dealing with disk crash

RAID: redundant array of independent disksStandard way of organizing disks and classifying the

reliability of multi-disk systemsGeneral methods: data duplication, parity, and error-

correcting codes (ECC)

RAID 0

No redundancyFailure causes data loss

Non-Redundant Disk Array Diagram (RAID Level 0)

open(foo) read(bar) write(zoo)

FileSystem

Mirrored Disks (RAID Level 1)

Each disk has a second disk that mirrors its contents

Mirrored Disks (RAID Level 1)

Writes go to both disks

+ Reliability is doubled

+ Read access faster

- Write access slower

- Expensive and inefficient

Mirrored Disk Diagram (RAID Level 1)

open(foo) read(bar) write(zoo)

FileSystem

Memory-Style ECC (RAID Level 2)

Some disks in array are used to hold ECC Using Hamming codes as the ECC

correct one bit error in a 4 bits code word requires 3 redundant bits.

Memory-Style ECC (RAID Level 2)

+ More efficient than mirroring

+ Can correct, not just detect, errors

- Still fairly inefficiente.g., 4 data disks require 3 ECC disks

Memory-Style ECC Diagram (RAID Level 2)

open(foo) read(bar) write(zoo)

FileSystem

Bit-Interleaved Parity (RAID Level 3)

One disk in the array stores parity for the other disksEnough to correct the error when the disk controller

tells which disk fails.

+ More efficient that Levels 1 and 2- Parity disk doesn’t add bandwidth

Parity Method

Disk 1: 1001Disk 2: 0101Disk 3: 1000Parity: 0100 (even parity: the number of

1’s is an even number)

How to recover disk 2?

Bit-Interleaved RAID Diagram (Level 3)

open(foo) read(bar) write(zoo)

FileSystem

Block-Interleaved Parity (RAID Level 4)

Like bit-interleaved, but data is interleaved in blocks

Block-Interleaved Parity (RAID Level 4)

+ More efficient data access than level 3

- Parity disk can be a bottleneck- Every write needs to write the parity disk.

- Small writes require 4 I/OsRead the old blockRead the old parityWrite the new blockWrite the new parity

Block-Interleaved Parity Diagram (RAID Level 4)

open(foo) read(bar) write(zoo)

FileSystem

Block-Interleaved Distributed-Parity (RAID Level 5)

Sort of the most general level of RAIDSpreads the parity out over all disks

+ No parity disk bottleneck

Block-Interleaved Distributed-Parity (RAID Level 5)

+ All disks contribute read bandwidth

– Requires 4 I/Os for small writes

Block-Interleaved Distributed-Parity Diagram (RAID Level 5)

open(foo) read(bar) write(zoo)

FileSystem