1. COT 5611 – Spring 2012 Operating Systems Design Principles
Dan C. Marinescu
Office: HEC 304
Office hours: M-Wd 5:00-6:00 PM

2. Lecture 23 – Wednesday April 4, 2012
Reading assignment:
Chapter 9 from the on-line text
Last time –
Atomicity
All-or nothing and before-or-after atomicity
Applications of atomicity
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3. Today - Cell storage versus journal storage - All-or-nothing atomicity - Before-or-after atomicity
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4. Cell storage versus journal storage
random-access memory and disk  a set of named, shared, and rewritable cells
hard to implement all-or-nothing semantics:
storing destroys old data
storing data reveals data to the later threads immediately
if the result consists of several output values, all of which should be exposed simultaneously it is harder to construct the all-or-nothing action
Journal storage:
associate with every named variable not a single cell, but a list of cells in non-volatile storage that represent the history of the variable
add a layer between the application and the cell storage; this layer appends a new prospective new value
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6. The state transitions of a journal storage system
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7. Read and write procedures: caller_id is the action identifier returned by NEW_ACTION
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8. Example: a thread has created a new record with: data_id=A, new_value=75, and client_id=1794. The procedure READ_CURRENT_VALUE will return value 24 for A and ignore versions aborted or pending.
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9. An all-or-nothing transfer using journal storage
Note: the transaction is not before-or-after
It checks if there are enough funds in the credit_account.
The order of steps is unconstrained
Problems: updates to version history and changes to the outcome must be all-or-nothing. But these can be done by overriding a single cell.
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10. Atomicity logs and journal storage
Log  An interleaved version of all variables; the information about the update of each data forms a record appended at the end of the log.
Easy access to a log, only the pointer to the last record is needed
Combine all-or-nothing atomicity of journal storage with the speed of cell storage. Two steps
Log  carry out the change in the journal storage
Install  change the cell storage by overriding the previous version of each record
The log is the authoritative record of the outcome of an action; the cell storage can be reconstructed using the log.
The log should reside in non-volatile memory.
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11. Types of logs
Atomicity log. Allows a crash recovery procedure to undo all-or-nothing actions that didn’t complete, or finish all-or-nothing actions that committed but that didn’t record all of their effects.
Archive log. Many uses for archive information: watching for failure patterns, reviewing the actions of the system preceding and during a security breach, recovery from application-layer mistakes fraud control, and compliance with record-keeping requirements.
Performance log. Most mechanical storage media have much higher performance for sequential access than for random access. Since logs are written sequentially, they are ideally suited to such storage media. When combined with a cache that eliminates most disk reads, a performance log can provide a significant speed-up.
4. Durability log. If the log is stored on a non-volatile medium (e.g., magnetic tape) that fails in ways and at times that are independent from the failures of the cell storage medium (e.g., magnetic disk) then the copies of data in the log are replicas that can be used as backup in case of damage to the copies of the data in cell storage. Any log that uses a non-volatile medium also helps support durability.
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12. Logging configurations
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13. Logging protocols
Reason for write-ahead-log  log append while install overwrites
Log record
Id of the all-or-nothing action performing the update
The do or redo action  component action that can perform the install if the system crashes before the install.
The undo action  component action that can reverse the effects if the system crashes during the install.
Four types of log records
BEGIN  NEW_ACTION writes this record and records the action id
CHANGE  written by the pre-commit phase
OUTCOME  written by the COMMIT or by the ABORT procedures
END  the final step of an action
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14. Example of log records
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15. Example: all-or-nothing TRANSFER with logging
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16. Recovery procedures for databases in volatile storage
We need recovery procedures in case of a system crash
The log is not affected as it resides on non-volatile memory
Abandon the in-core database and all all-or-nothing actions in progress
Two steps:
Backwards scan the log and identify all actions with an OUTCOME record showing that the action has been COMMITTED, call them winner actions
Forward scan the log and identify REDO actions of every winner whose OUTCOME record shows that the action has been COMMITTED. Reinstall all committed action values in cell storage.
The recovery procedure should be idempotent  if the system crashes during recovery we should be able to start again.
The ABORT procedure should also be idempotent
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18. Recovery for databases on non-volatile storage (disk)
Large databases cannot be kept in main memory.
Access to disk (reads and writes) is slower
Installs survive a system crash and we are faced with new problems.
A. The recovery procedure must reverse the effects of pending AONA that have installed changes.
B. The entire database must be reinstalled, time consuming for large databases!!
We assume a write-through cache to avoid complications as the multi-level memory manager may defer writing.
To address A:
Backward log scan phase look for losers (rather than winners) – a loser is an action in progress at the time of the crash, has no END record.
UNDO the CHANGE record of a loser action.
Roll back all INSTALLS performed by losersas if the actions of losers never occurred
Perform a forward log scan and perform the REDO of all COMMITTED actions
Add an END record to all losers
Blind write overwrite a data without reference to its previous value. Idempotent operation for UNDO and REDO actions during the recovery. Necessary to make recovery idempotet.
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20. Undo logging or rollback recovery
To address B  undo logging or rollback recovery
Additional requirement to avoid to REDO any INSTALLs  perform all INSTALLs before logging it in the OUTCOME record  all INSTALLS are in the non-volatile storage, no need to REDO them
UNDO only the INSTALLs of losers and skip the forward scan
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21. Summary of atomicity logging
Log to journal storage before installing in cell storage
For non-volatile storage
Only UNDO the INSTALLs of incomplete actions if the INSTALL is carried out before logging the OUTCOME record
Only REDO the installs of incomplete actions when logging is done before INSTALL
Otherwise do both UNDO and REDO.
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22. Before-or-after atomicity
Simple serialization  assume each transaction has a unique t_id
t_id is from a compact set of integers; at initialization time the system creates a transaction with t_id=0.
The rule transaction with t_id=n must wait before reading or writing any data that transaction with t_id=n-1 has either committed or aborted
Produces correct results
It is too conservative, it does not allow any parallelism
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24. A transaction needs to know the version history of all variables it affects, it does not need to wait for previous one to complete if they are disjoint, e.g. t4 and t3
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25. The mark-point discipline
A transaction
identifies the data it intends to modify - marks it;
creates a pending version of the variable
mark point  the instance when a transaction has finished updating the marking all variables it intended to change
announces that it has passed its marking point  sets a flag in the OUTCOME record
A transaction has to wait until all preceding ones have reached the marking point before it can begin reading variables marked by preceding transactions
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26. Skip versions of a variable created by transaction with t_id > id_current_transaction Wait for pending versions of the variable created by transactions with t_id < id_current_transaction
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27. Marking becomes a sequence on calls to NEW_VERSION and WRITE_VALUE
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28. The system should be initialized with a call to NEW_OUTCOME_RECORD to ensure that there is a previous transaction as required by BEGIN_TRANSACTION
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29. More on mark-point discipline
The result is guaranteed to be the same as if the transactions were executed sequentially.
Potential interaction of before-or-after and all-or-nothing atomicity if pending versions survive a system crash at the restart all PENDING transactions must be identified and marked as ABORTED
The discipline never creates deadlock  there is no circular WAIT, a transaction must wait only for preceding ones.
If a transaction waits to announce its mark point after the COMMIT or ABORT this becomes simple serialization.
Two possible errors:
A transaction calls NEW_VERSION after announcing its mark_point
WRITE_VALUE attempts to write a value for a variable for which a new version has never been created
Example: the TRANFER transaction with using mark point to achieve before-or-after atomicity
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31. Optimistic before-or-after atomicity
Assume that the interference between concurrent transactions is unlikely and allow them to proceed; if interference occurs recover e.g., abort and restart.
Increases concurrency.
Read-capture startegy
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